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Dictionary.jpg Definitions & Glossary - Sleep Apnea and CPAP Terms

A, B, C

Abdominal Movement - Movement of the abdomen, which reflects breathing effort and movement of the diaphragm. This is typically measured by the placement of a sensor band around the chest. Abdominal movement is recorded in diagnostic sleep studies. This measurement is used in differentiating the type of sleep apnea.

Acidosis - A pathologic condition resulting in the accumulation of acid or loss of base in the body; Respiratory acidosis is the state of excess retention of carbon dioxide.

Actigraph - An instrument for the measurement of body movement.

Adult Respiratory Distress Syndrome (ARDS) - Indicates respiratory failure with life threatening distress and hypoxemia, associated with acute pulmonary injuries

Advanced Sleep Phase Syndrome - A circadian rhythm disorder in which phases of the daily sleep/wake cycle are advanced with respect to clock time. The sleep phase occurs well ahead of conventional bedtime, creating a tendency to fall asleep or wake up too early.

Airflow Limitation (Flow Limitation) - Partial closure of the upper airway with reduced airflow.

Alpha Rhythm - An EEG brainwave produced when an individual is in a relaxed awake condition with their eyes closed. The frequency is 8-13 Hz in human adults, and is predominately present in the occipital lobe of the brain. It indicates the awake state in most normal individuals.

Alpha Sleep - Sleep in which alpha activity occurs with sleep EEG patterns.

Alpha-Delta Sleep - On rare occasions, Delta brainwaves (deep sleep) are mixed with alpha brainwaves (relaxed wakefulness). Alpha-delta sleep occurs with conditions such as arthritic pain, stimulant use and thyroid imbalance.

Ambulatory Monitor - Portable system used for the continuous recording of multiple physiological variables during sleep.

Antidepressant - A class of medications most commonly used in the treatment of depression. Most antidepressants also reduce REM sleep. In sleep disorders, they are used to control cataplexy, hypnagogic hallucinations, and sleep paralysis.

Apnea - Cessation of breathing for 10 or more seconds during sleep. There are two basic types of sleep apnea: Obstructive Apnea is caused by a closure of the air passage despite efforts to breathe; Central Apnea is a lack of effort to breathe. Obstructive Sleep Apnea is by far the most common type. True apnea during wakefulness is extremely rare.

Apnea/Hypopnea Index - (AHI) (RDI or Respiratory Disturbance Index) The frequency of abnormal respiratory events per hour of sleep. These events are classified as Apneas or Hypopneas. Apnea is when breathing (airflow) stops for 10 seconds or more. Hypopnea is a partial blockage of airflow resulting in arousal and a possible drop in oxygen level. An AHI of 45 would indicate that the patient is experiencing complete or partial airflow blockage 45 times per hour.

Apnea Index - The frequency of apneas per hour of sleep.

Arousal - An abrupt change from sleep to wakefulness, or from a "deeper" stage of non-REM sleep to a "lighter" stage.

Arousal Disorder - A parasomnia disorder presumed to be due to an abnormal arousal function. The classical arousal disorders are sleepwalking, sleep terrors, and confusional arousals.

Arousal Threshold - The ease with which a sleeping person is awakened at a particular time.

Arrhythmia - An absence or irregularity of the heart rhythm caused by disturbances of the transmission of electrical impulses through the cardiac tissue.

Auto Adjusting Continuous Positive Airway Pressure Device (Auto-CPAP or SmartPAP) - See CPAP. A type of CPAP machine that monitors changes in the individual's breathing and compensates automatically by making the appropriate adjustment in pressure.

Automatism - An automatic action; especially any action performed without apparent intention or awareness.

Awakening - The return to the polysomnographically defined awake state from any of the non-REM sleep stages or REM sleep: characterized by alpha and beta waves, rise in tonic EMG, voluntary eye movements and eye blinks.


Basic Sleep Cycle - See Sleep Cycle.

Benzodiazepines - Class of sedative medication commonly used to treat anxiety and insomnia.

Beta Activity - Brain waves seen in alert wakefulness which have a frequency greater than 13 Hz (Hertz).

Bi-Level Positive Airway Pressure (Bi-level or BiPAP) - See CPAP. A CPAP device providing two measured pressure levels of continuous airflow: one level for inhalation and a lower level for exhalation.

Biological Clock - A term applied to the brain process that regulates 24-hour fluctuations in body temperature, hormone secretion, and a host of other bodily activities. Its most important function is to foster the daily alternation of sleep and wakefulness. The biological clock is housed in a pair of tiny bilateral brain areas called the suprachiasmatic nuclei.

Biological Rhythms (Circadian Rhythms) - An innate daily fluctuation of physiological or behavioral functions, including sleep-wake states generally tied to the 24-hour daily dark-light cycle. Sometimes occurs at a measurably different periodicity (e.g. 23 or 25 hours) when light-dark and other time cues are removed.

Blood Oxygen desaturation - a reduction in the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen.

Blood Oxygen Saturation (SO2) - commonly referred to as "sats", measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. At low partial pressures of oxygen, most hemoglobin is deoxygenated. Normal blood oxygen saturation levels for most healthy persons averages around 95% to 100%. A pulse oximeter is used to measure blood oxygen levels.

Body Position - In sleep studies, four sleep positions are identified; back, left side, right side or abdomen. Some tests also indicate if a patient is sitting up. The amount of time spent sleeping in each position and the occurrence of respiratory events in a particular position are tabulated. Body position is recorded in both diagnostic sleep studies and CPAP titration studies.

Bradycardia - A heart rhythm with a rate below 60 beats per minute in a human adult.

Brain Waves - Spontaneous electrical activity of the brain studied by method of electroencephalography (EEG).

Brick - Slang term for CPAP machines that record limited data. "Power brick" may be used for the external power supply that goes between the CPAP machine and the electrical outlet.

Bruxism (Toothgrinding) - Grinding one's teeth while asleep. This occurs at some time in approximately 70% of people; most have no noticeable side effects. However, 5% of victims develop symptoms such as tooth wear, jaw pain and headaches. Episodes of grinding are more severe after stressful days.


Cardiac Arrest - Sudden cessation of heart beat.

Cardiovascular - Pertaining to the heart and blood vessels.

Central Nervous System (CNS) - The brain and spinal cord.

Central Sleep Apnea - A period of at least 10 seconds without airflow, during which no respiratory effort is evident. Central sleep apnea (CSA)

Cheyne–Stokes respiration (CSR) - An abnormal pattern of breathing most often associated with congestive hearth failure (CHF) characterized by progressively deeper, and sometimes faster, breathing followed by a gradual decrease that results in a temporary stop in breathing called a central apnea at the nadir of respiratory flow. The pattern repeats, with each cycle usually taking 30 seconds to 2 minutes. Note, Resmed machines flag periodic breathing as CSR and in most cases it not CSR. Please see Periodic Breathing.

Chronotherapy - Treatment of a circadian rhythm sleep disorder by systemically changing sleeping and waking times to reset the patient's biological clock.

Circadian - Any periodicity that is near but not necessarily exactly 24 hours.

Circadian Rhythms - See Biological Rhythms

Claustrophobia - Fear of being confined in any enclosed space, as in a locked room.

cmH2O (centimeter of water) is a unit of measurement used to measure CPAP air pressure, typically via a manometer. It is defined as the pressure exerted by a column of water of 1 cm in height at 4 °C (temperature of maximum density) at the standard acceleration of gravity.

Compliance - Adhering to a regimen of treatment, such as CPAP therapy.

Continuous Positive Airway Pressure (CPAP) Machine - Medical device used to treat sleep apnea. This apparatus provides a highly effective, non-invasive therapy that eliminates blockages and prevents collapse of the upper airway by gnerating a prescribed level of air pressure that maintains airway patency during sleep. Air pressure is delivered through a hose to a mask that fits over the nose, or both nose and mouth. The mask is secured on the face by headgear that is worn over the head. The appropriate air pressure level is determined during a "CPAP titration" sleep study. The complete system consists of a programmable pressure generator, tubing, mask and headgear. Sometimes referred to as nCPAP (nasal Continuous Positive Airway Pressure).

Confusional Arousals - Also called sleep drunkenness or excessive sleep inertia, this disorder is an extreme example of the slowness most people feel upon awakening. People with confusional arousals respond poorly to commands or questions, and have major memory impairment of things that happened a short time before.

CPAP Pressure - Amount of pressure needed to maintain an open airway in a sleep apnea patient being treated with CPAP, expressed in centimeters of water (cm H20). The positive pressure may range from 5 to 20 cm H20. Different patients will require different pressures. This value is determined in a CPAP titration study.

Cycle - The beginning of expiration or EPAP. A bilevel machine "cycles" from IPAP to EPAP.

D, E, F

Deep Sleep (Delta Sleep) (Slow Wave Sleep) - In sleep studies, refers to combined non-REM sleep stages 3 and 4.

Delayed Sleep Phase Syndrome - A circadian rhythm disorder, which in the daily sleep/wake cycle, is delayed with respect to clock time. Accordingly, the sleep phase occurs well after the conventional bedtime. Usually associated with difficulty getting up in the morning.

Delta Sleep (Deep Sleep) (Slow Wave Sleep) - Sleep stages 3 and 4 in which EEG delta waves are prevalent.

Delta Waves - Delta waves have a frequency of 2 cycles per second (cps) or slower and amplitudes greater than 75 microvolts peak to peak (the difference between the most negative and positive points of the wave). Also known as Delta Activity.

Diagnostic Sleep Study - Continuous monitoring of several physiological activities in a sleeping individual. Usually carried out to determine the absence or presence of a specific sleep disorder. A diagnostic sleep study can be performed in a sleep disorders center or in a patient's home with portable recording equipment.

Diaphragm - The large, concave muscle attached to the rib cage at bottom of the chest (top of the abdomen). Inhalation occurs when the diaphragm contracts. Exhalation is usually passive as the muscle relaxes.

Diurnal - Active and wakeful in the daytime, as opposed to active in the nighttime.

DME - (Durable Medical Equipment) - Devices such as wheelchairs and walkers, which are prescribed on the order of a physician. This includes CPAP and Bi-Level machines. Sometimes referred to as HME (Home Medical Equipment).

Drowsiness, Drowsy - A state of quiet wakefulness that typically occurs prior to sleep onset. If the eyes are closed, diffuse and slowed alpha activity usually is present, which then gives way to early features of stage 1 sleep.

Dyssomnias - A class of sleep disorders that produce either insomnia or excessive sleepiness.


Electrocardiography (EKG) (ECG) - Method of measuring the electrical activity of the heart. EKG is continuously recorded in both diagnostic sleep studies and CPAP titration sleep studies.

Electrodes - Small devices that transmit brain waves or other biological electrical signals from a patient to a polysomnograph machine, where the signal is amplified and displayed.

Electroencephalogram (EEG) - A recording of the electrical activity generated by the brain. Brain wave frequencies are expressed in hertz (Hz) or cycles per second (cps), and amplitude is expressed in microvolts. Characteristic frequency and amplitude patterns of the activity define which stage of sleep the patient is experiencing. Brain waves are recorded in both diagnostic and CPAP titration sleep studies.

Electromyogram (EMG) - A recording of the electrical activity of the muscles. The absence of very low level of EMG activity indicates the presence of REM sleep. Very high levels can help identify periods of wakefulness. Non-invasive EMG is recorded in both diagnostic and CPAP titration sleep studies.

Electrooculogram (EOG) - A recording of the movements of the eyes. If rapid eye movements are detected during sleep, the subject is in REM sleep, which is the state in which vivid dreaming takes place. EOG is recorded in both diagnostic and CPAP titration sleep studies.

EPAP - (Expiratory Positive Airway Pressure) - The pressure prescribed for the expiratory (breathing out) phase of an individual on Bi-level CPAP therapy for OSA (obstructive sleep apnea).

Epidemiology - The scientific discipline that studies the incidence, distribution, and control of disease in a population. Includes the study of factors that affect the progress of an illness, and, in the case of many chronic diseases, their natural history.

Epoch - A standard 30-second page of the sleep recording that is assigned a sleep stage designation; occasionally, for special purposes, longer or shorter epochs are scored.

Epworth Sleepiness Scale - An index of sleep propensity during the day as perceived by patients, and derived from the answers to 8 questions.

Esophageal Pressure - A measurement used to determine respiratory effort and by inference, airway resistance. Considered an invasive measure, it is sometimes used in polysomnographic testing that is conducted in sleep disorders centers.

Excessive Daytime Sleepiness (EDS, Somnolence, Hypersomnia) - A subjective report of difficulty in maintaining the awake state, accompanied by a ready entrance into sleep when the individual is sedentary; may be quantitatively measured by use of subjectively defined rating scales of sleepiness.

Expiratory Phase - The phase of the breathing cycle in which air is expelled.


Fatigue - A feeling of tiredness or weariness usually associated with performance decrements.

Fiberoptic Nasopharyngoscope - A flexible fiberoptic scope used to examine the nasal passages, pharynx, hypopharynx and larynx.

Flattening Index - A number that indicates the amount of airflow limitation caused by partial closure of the upper airway. 0.3 indicates an open airway, 0.15 is mildly obstructed, 0.1 is severely limited airflow, and 0.0 reflects a totally closed airway. Flattening Index is used to identify a condition known as Upper Airway Resistance Syndrome (UARS), and is continuously recorded in some diagnostic sleep studies and CPAP titration studies.

Flow Limitation (FL) - Partial closure of the upper airway, which impedes the flow of air into the lungs.

Forbidden Zone - Period of strongest clock-dependent alerting, usually in the evening. Prevents falling asleep.

Fragmentation (pertaining to Sleep Architecture) - The interruption of a sleep stage due to the transition to a lighter stage, or to wakefulness, leading to disrupted sleep cycles.

G, H, I

GABA (Gamma-Amniobutyric Acid) - A major inhibiting compound (neurotransmitter) in the brain, which is considered to be involved in muscle relaxation, sleep, diminished emotional reaction and sedation. GABA is released in the greatest amount from the cerebral cortex during slow wave sleep.

Gastroesphageal Reflux Disease (GERD) - The flow of stomach acid upwards into the esophagus, which can cause arousals and disrupt sleep.

Genioglossus tongue advancement - Surgical treatment sometimes used for sleep apnea or snoring. This is designed to improve the airway behind the base of the tongue. The genioglossus, which is the main tongue muscle, relaxes during sleep, often allowing the tongue to fall into the airway. This muscle attaches to the middle of the lower jaw. If a segment of bone containing this muscle is pulled forward and stabilized, it can open the airway space behind the tongue. This procedure does not move the teeth or jaw. It is performed under local intravenous sedation or general anesthesia and requires a one or two day hospital stay


Habitual Snorers - Individuals who snore nearly every night.

Heart Rate or beats per minute (bpm) - Pace or speed of the heart measured in beats per minute; 60-80 is generally considered normal. Heart rate is continuously monitored in both diagnostic sleep studies and CPAP titration studies.

Hertz (Hz) - A unit of frequency measuring cycles per second (cps). A measurement used in describing sleep EEG brain waves.

Histogram - A graph showing frequency distributions.

HME - (Home Medical Equipment) - Devices such as wheelchairs and walkers, which are prescribed on the order of a physician for use in the home. This includes CPAP and Bi-Level machines. Sometimes referred to as DME (Durable Medical Equipment).

Humidification - Adding moisture to the airflow as an adjunct to CPAP (Continuous Positive Airway Pressure) therapy in the treatment of obstructive sleep apnea (OSA). Humidification is added to the CPAP device by diverting the airflow over a cool or heated water reservoir (humidifier) to prevent the upper airway from drying out.

Hyoid Suspension - Surgical procedure sometimes used in the treatment of sleep apnea or snoring. It is designed to improve the airway behind the base of the tongue. The hyoid bone is located in the neck where some tongue muscles attach. If the hyoid bone containing these muscles is pulled forward in front of the voice box, it can open the airway space behind the tongue. This is performed under local intravenous sedation or general anesthesia, and requires a one or two day hospital stay.

Hypercapnia - An excess of carbon dioxide in the blood.

Hyperirritability - Extreme irritability; often seen in sleep deprived subjects.

Hypersomnia - Sleeping for uncharacteristically long periods of time.

Hypersomnolence - Excessive daytime sleepiness.

Hypertension - High blood pressure.

Hypnic Jerk - Sensation of falling, and then "jerking" awake just as one drifts to sleep. It is a normal sensation experienced by many people.

Hypnophobia - Morbid fear of falling asleep.

Hypnotics - Sleep-inducing medications; sleeping pills.

Hypopharynx - The lowermost portion of the pharynx leading to the larynx and esophagus.

Hypopnea - An episode of diminished breathing during sleep, caused by a partial airway obstruction, and resulting in arousal. Usually accompanied by oxygen desaturation. Hypopneas may be just as serious as apneas and have the same troublesome effects. Hypopnea

Hypoventilation - Reduced rate and depth of breathing.

Hypoxemia - Lack of an adequate amount of oxygen in the blood.

Hypoxia - A deficiency of oxygen reaching the tissues of the body.


Imidazopyridines - Newest class of medications that induce sleepiness. They are short acting and few side effects have been found with this class of hypnotics. (Zolpidem, trade name Ambien, is in this class).

Inappropriate Sleep Episodes - Periods of sleep that are not planned and often occur in an unsafe situation (i.e., while driving). These episodes are always due to sleep deprivation.

Insomnia - Difficulty with falling asleep or staying asleep.

Inspiratory Phase - Part of the breathing cycle during which air is inhaled.

Intermediary Sleep Stage - A term sometimes used for non-REM stage 2 sleep. See Deep Sleep, Light Sleep.

Invasive - Refers to a medical procedure in which a bodily orifice or the skin must be penetrated for the purpose of collecting data, or for diagnosing or treating a disorder.

IPAP - Inspiratory Positive Airway Pressure. The pressure prescribed by a physician for the inspiratory phase on a Bi-level CPAP device, used in the treatment of OSA.

J, K, L

Jet Lag - A disturbance induced by a major rapid shift in environmental time during travel to a new time zone. Symptoms include fatigue, sleep and impaired alertness.


K-Alpha - A type of micro-arousal where a K complex is followed by several seconds of alpha rhythm.

K Complex - An EEG waveform most commonly found in stage 2 sleep. It has a well-delineated negative sharp wave that is immediately followed by a slower positive component.


Large Leak (LL) - Any leak that exceeds 25 liters per minute

Laser Assisted Uvuloplasty (LAUP) - Surgical treatment usually used for simple snoring or mild obstructive sleep apnea. This procedure is designed to open the airway behind the palate. It requires multiple procedures where the laser reduces tissue of the palate and the area heals by scarring. It is an outpatient procedure performed under local anesthesia.

Leak Rate - The liters per minute detected as a leak by a machine from the entire system (machine, connections, hose and mask).

Leg Movement - Leg movements are recorded in diagnostic sleep studies to test for Periodic Limb Movement Disorder.

Letter of medical necessity (LMN) - Special document that is a certification by a physician that the prescribed item(s) is medically indicated, reasonable and necessary with reference to the standards of medical practice and treatment of the patient's condition.

Light-Dark Cycle - The periodic pattern of light (artificial or natural) alternating with darkness.

Light Sleep - A common term used to describe non-REM sleep stage 1, and sometimes, stage 2.

Light Therapy - Used to treat SAD (Seasonal Affective Disorder) and other conditions. Exposing the eyes to light of appropriate intensity and duration at the appropriate time of day to affect the timing, duration and quality of sleep.

Limit-Setting Sleep Disorder - Occurs when a child stalls or refuses to go to bed, and subsequently does not receive enough sleep.

Linear Sleepiness Rating Scale - A measure of subjective sleepiness. The scale consists of a horizontal line, 100 mm in length on which the right extreme is labeled "Very Sleepy" and the left extreme is labeled "Very Wide Awake."

M, N, O

Macroglossia - Large tongue; usually a congenital disorder (present at birth). May result from other medical conditions such as acromegaly.

Manometer is a device for measuring pressure. For CPAP machines, manometers typically use cmH2O as a unit of measure.

Marijuana - Parts or extracts of the hemp plant, which contain an active ingredient called tetrahydrocanabinol (THC) that is euphorogenic. Some people use marijuana at bedtime as an aid to falling asleep.

Maxillofacial - Pertains to the jaws and face.

Maxillomandibular advancement - This surgical procedure opens the airway behind the palate as well as behind the base of the tongue. The operation cuts the bone of the upper and lower jaw and pulls these structures forward. This is performed under general anesthesia and requires a two-day hospital stay. This type of treatment is usually done if previous procedures have not completely improved the obstructive breathing episodes and the patient has persistent symptoms of daytime sleepiness and fatigue.

Melatonin - A hormone secreted by the pineal gland in the brain. Melatonin has been reported to have hypnotic properties, leading some to suggest that melatonin, which is released at night, may be an endogenous sleep inducer.

MESAM 4 - Type of portable recording device used in diagnostic sleep studies.

Microsleep - Very brief episode of sleep that occurs in the midst of ongoing wakeful activity. Microsleeps are associated with sleep deprivation, drowsiness and automatic behavior.

Mixed Sleep Apnea - Combination of central and obstructive apnea.

Monocyclic - Having a single major sleep period and a single major wake period in one 24-hour day.

Motor Activity in Sleep - Any muscular movement during sleep.

Movement Arousal - A body movement associated with an EEG pattern of arousal or a full awakening.

Motor Atonia - Absence of muscle activity during sleep, a normal phenomenon during REM sleep.

Multiple Sleep Latency Test (MSLT) - The standard test used to quantify the overall daytime sleep tendency by measuring the speed of falling asleep (sleep latency) usually in 5 tests carried out at two-hour intervals. This test also helps in the diagnosis of narcolepsy. Patients with narcolepsy often go directly from wakefulness to REM, which can be evaluated with the MSLT.

Muscle Tone - The amount of tension in a muscle.

Myoclonus - Muscle contractions in the form of abrupt "jerks" or twitches generally lasting less than 100 milliseconds. The term should not be confused with the periodic limb movements of sleep that characteristically have a duration of 0.5-5 seconds.


Nap - A short period of sleep generally obtained at a time separate from the daily major sleep period.

Narcolepsy - A sleep disorder characterized by excessive sleepiness, cataplexy, sleep paralysis, hypnagogic hallucinations, and an abnormal tendency to pass directly into REM sleep from wakefulness. It was recently found to be caused by an abnormal gene in the brain.

Nasal Airflow/Nasal Ventilation - A recording of the respiratory cycle by measuring inspiratory and expiratory airflow. In diagnostic sleep studies it is referred to as nasal ventilation or respiratory airflow. It is measured through heat or pressure changes at the nares. This value is usually recorded in both diagnostic sleep studies and CPAP titration sleep studies.

National Commission on Sleep Disorders Research - Created by the U.S. Congress in 1990, the commission conducted a comprehensive study of the social and economic impact of sleep disorders in America, and made recommendations based on its findings to Congress in January of 1993.

Neurology - A specialized branch of medicine that deals specifically with the nervous system and its diseases.

Neurotransmitters - Endogenous chemical components that are released from axon terminals of one neuron and transmit the signal to the next neuron by combining with its receptor molecules. Examples of neurotransmitters that appear to be important in the control of sleep and wakefulness include: norepinephrine, serotonin, acetylcholine, dopamine, adrenaline and histamine. The process of neurotransmission may be inhibited, modulated, or enhanced by other chemical mediators within the brain, or by exogenous pharmaceuticals.

Nightmare - An unpleasant and/or frightening dream that usually awakens a person from REM sleep. Occasionally called a dream anxiety attack, it is not synonymous with a night (sleep) terror.

Night Terrors - Also known as sleep terrors, or pavor nocturnus. Usually a disorder of childhood, characterized by a piercing scream, signs of intense fear, and unresponsiveness to other people. It is not a REM stage nightmare, but an incomplete arousal from slow wave sleep. If awakened during a night terror, the individual is usually confused and does not remember details of the event. Night terrors are different from nightmares in that if an individual is awakened during a nightmare, he or she functions well and may have some recall of the nightmare.

Nyctophobia - Morbid fear of the night and darkness.

Nocturia - Excessive, and often frequent, urination during the night.

Nocturnal - "Of the night;" pertains to events that happen during sleep or the hours of darkness.

Nocturnal Confusion - Episodes of delirium and/or disorientation close to or during nighttime sleep; often seen in victims of Alzheimer's disease and more common in the elderly.

Nocturnal Enuresis (Bedwetting) - The release of urine while asleep.

Non-Invasive - Medical procedure that does not penetrate the skin or a body cavity.

NREM Sleep (Non-Rapid Eye Movement, Non-Rem) - All sleep stages other than REM sleep; made up of sleep stages 1 through 4. Characterized by a slowing of brain waves and some physiological functions. A state that lacks the visible motility of rapid eye movements and twitches. See Sleep Stages.

NREM-REM Sleep Cycle (synonymous with Sleep Cycle) - A period during sleep composed of a NREM sleep episode followed by a REM sleep episode. Each NREM-REM sleep couplet is equal to one cycle; with the average duration in adults being 90 minutes. An adult sleep period of 6.5-8.5 hours generally consists of four to six cycles.

NREM Sleep Intrusion - A brief period of NREM sleep patterns appearing in REM sleep, outside of its usual sleep cycle position.

NREM Sleep Period - The NREM sleep portion of NREM-REM sleep cycle. See Sleep Cycle, Sleep Stages.

NREM Sleep - Consists of sleep stages 1 through 4, and characterized by a decrease of mental activity. A state that lacks the visible motility of rapid eye movements and twitches, and exhibits a different EEG pattern than REM sleep. See Sleep Stages.


Obesity-Hypoventilation Syndrome - A term applied to obese individuals who have abnormally slow and shallow respiration (hypoventilation) during wakefulness, resulting in an increased level of carbon dioxide in the blood.

Obstructive Apnea (OA) - A cessation of breathing during sleep for 10 seconds or more due to complete closure (collapse) of the throat.

Obstructive Hypopnea - Periodic, partial closure of the throat during sleep resulting in reduced air exchange. The medical consequences of this partial closure can be as severe as sleep apnea (full closure of throat) Usually characterized by snoring, excessive daytime sleepiness, and other symptoms of fatigue

Obstructive Sleep Apnea (OSA) - Repetitive cessation of breathing during sleep for 10 seconds or more due to complete closure (collapse) of the throat. Usually characterized by snoring, excessive daytime sleepiness, and other symptoms of fatigue. Obstructive sleep apnea (OSA)

Ondine's Curse - Condition in which the respiratory center in the brain is unable to stimulate breathing in response to an increased amount of carbon dioxide in the blood. Ondine's Curse is synonymous with central alveolar hypoventilation and typically worsens during sleep.

Optimum Sleep - The average amount of sleep needed every night by any given individual. Most adults require about 7 to 8.5 hours per night, although this number is subjective, and may vary from person to person.

Oscillograph - A device that displays ongoing physiological patterns (EEG) or other variables usually tracing on moving paper or a CRT screen.

Oxygen Desaturation - Less than normal amount of oxygen carried by hemoglobin in the blood. A value below 90% is considered abnormal.

Oxygen Saturation - A measure of oxygen carried by hemoglobin in the blood. Normal values range from 90% to 100%. An important indicator of sleep disordered breathingthat is directly affected by the degree of throat closure (partial vs. complete) and its duration. Oxygen saturation is recorded in both diagnostic sleep studies and titration studies.

Oximeter - Medical device used to provide estimates of arterial oxygen saturation (SaO2) by utilizing selected wavelengths of light to noninvasively determine the saturation of oxyhemoglobin (SpO2).

Oximetry - Test used to continuously monitor the oxygen saturation of arterial blood from am oximeter sensor.

P, Q, R

Periodic Breathing (PB) - Periodic breathing is defined as three or more episodes of central apnea lasting at least 3 seconds, separated by no more than 20 seconds of normal breathing.

Pressure Support - Pressure support (PS) in positive air pressure therapy, is the difference between inspiratory positive air pressure (IPAP) and expiratory positive pressure (EPAP). For example if IPAP is set at 12 and EPAP is set at 8 the PS is 12 - 8 = 4.0.

Pulse Oximetry - Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin at the fingertip or earlobe.


Quiet Sleep - The term frequently used instead of NREM sleep in describing the sleep of infants. Quiet/NREM sleep comprises 50% of a newborns' total sleep time.


Radiofrequency - Electromagnetic radiation in the frequency range 3 kilohertz (kHz) to 300 gigahertz (GHz). It is generally considered to include microwaves and radio waves. Microwaves occupy the spectral region between 300 GHz and 300 MHz, while RF or radio waves include 300 MHz to 3 kHz. The primary health effects of radiofrequency energy are considered to be thermal. The absorption of radiofrequency energy varies with frequency.

Radiofrequency (RF) Procedure (Somnoplasty) - A procedure for treating nasal obstruction, snoring and in some cases, sleep apnea. This procedure uses radio wave energy to reduce snoring and the size of a redundant soft palate. These techniques are performed under local anesthesia as an out-patient procedure.

Rapid Eye Movement Sleep - See REM Sleep, Sleep Stages.

REM Density - The frequency of eye movements per unit of time during REM sleep.

REM-Associated Disorders - Sleep disturbances that occur in REM sleep.

REM Motor Atonia - The suppression, or paralysis, of voluntary muscles during REM sleep. These muscles become completely flaccid and limp.

REM Sleep (Rapid Eye Movement Sleep) - The sleep stage in which vivid dreaming occurs; identified by the occurrence of rapid eye movements under closed eyelids, motor atonia and low voltage EEG patterns. Also associated with bursts of muscular twitching, irregular breathing, irregular heart rate, and increased autonomic activity.

REM Sleep Behavior Disorder - A disorder in which REM motor atonia is partially or completely absent. People with this disorder are able to move their muscles and act out their dreams. The behaviors may include punching, kicking, leaping and running from the bed.

REM Sleep Episode - The REM sleep portion of a NREM-REM sleep cycle. There are typically 4-6 REM sleep episodes during an entire sleep period. Early episodes may be only several minutes in duration. Later REM episodes are usually longer, lasting 20 to 60 minutes.

REM Sleep Intrusion - A brief interval of REM sleep appearing out of its usual positioning in the NREM sleep cycle.

REM Sleep Latency - The interval from sleep onset to the first appearance of REM sleep.

REM Sleep Onset - The designation for the first epoch of a REM sleep episode.

REM Sleep Percent - The proportion of total sleep time occupied by REM sleep.

REM Sleep Rebound - A compensatory increase in REM sleep following a reduction of REM Sleep Percent. An increase in time, frequency and density of REM sleep episodes.

Respiratory Care Practitioner (RCP) - A licensed health care professional specifically trained in cardiopulmonary assessment, diagnostics, therapy administration, and patient education, including the identification and treatment of sleep disorders.

Respiratory Disturbance Index (RDI)(Apnea/Hypopnea Index) - The frequency of abnormal respiratory events per hour of sleep. These events are classified as Apneas or Hypopneas. Apnea is when breathing (airflow) stops for 10 seconds or more. Hypopnea is a partial blockage of airflow resulting in arousal and a possible drop in oxygen level. An RDI of 45 would indicate that the patient is experiencing complete or partial airflow blockage 45 times per hour.

Restless Legs Syndrome - The sleep disorder characterized by tingling, creeping, crawling, or aching sensation in the legs that tends to occur when an individual is not moving. There is an almost irresistible urge to move the legs that relieve the sensations. Inability to remain at rest can result in severe sleep disturbance.

Restlessness (Referring to Quality of Sleep) - Persistent or recurrent body movements, arousals, or brief awakenings in the course of sleep.

S, T, U

Seasonal Affective Disorder (SAD) - A mood disorder occurring in the winter months characterized by diminished energy, hypersomnia, overeating and depressed mood. Exposure to bright light in the morning hours may be effective in alleviating or decreasing symptoms.

Sedatives - Drugs that tend to calm, reduce nervousness or excitement, and foster sleep onset.

Sedentary Situation - Does not require physical activity, e.g. working at a desk, sitting in a meeting or in a theater, watching television.

SensAwakeTM (SA) - SensAwakeTM is a Fisher & Paykel technology that uses flow to continuously monitor patients’ breathing patterns. It senses the change from breathing patterns associated with sleep to those associated with awake states. Once it detects a transition from sleep to awake it promptly reduces that pressure delivered to the patient.

Septoplasty - Surgery on the nasal septum (the partition that divides the nasal passage).

Serotonin - A neurotransmitter in the brain that modulates mood, appetite, sexual activity, aggression, body temperature and sleep.

Shiftwork - Working during times other than the conventional daytime hours of 9:00 a.m. to 5:00 p.m.

Siesta - A custom in many cultures of napping in the afternoon, usually after the noon meal. This allows people in tropical climates to avoid the hottest part of the day; it also takes advantage of the body's normal post-lunch (postprandial) dip in alertness.

Sleep - Overall state in which an individual rests quiescently in a recumbent position, disengages from the environment, and become unresponsive to stimuli.

Sleep Apnea - Cessation of breathing for 10 or more seconds during sleep. There are two basic types of sleep apnea: Obstructive Apnea is caused by a closure of the air passage despite efforts to breathe; Central Apnea is a lack of effort to breathe. Obstructive Sleep Apnea is by far the most common type.

Sleep Architecture - The sequence and duration of each sleep stage and awakening during a sleep period. Often displayed in the form of a histogram.

Sleep Cycle - The progression through an orderly succession of sleep states and stages. In a healthy adult, the first cycle is always initiated by going from wakefulness to non-REM sleep. The first REM period follows the first period of non-REM sleep to complete the first sleep cycle. The two sleep states continue to alternate throughout the night with an average cycle period of about 90 minutes. A full night of normal human sleep will usually consist of 4-6 non-REM/REM sleep cycles.

Sleep Debt - The result of recurrent sleep deprivation that occurs over time, when an individual does not obtain a sufficient amount of restorative daily sleep. Sleep debt is like a monetary debt; it must be paid back at some time. The larger the sleep debt, the stronger the tendency to fall asleep. This accumulation of "lost sleep" may contribute to a decreased quality of life, the onset of related health problems, and the increased risk of injury and/or accident. See Sleep Deprivation.

Sleep Deprivation - An acute or chronic lack of sufficient sleep, which causes a person to feel unrefreshed during wakefulness.

Sleep Disorders - A broad range of illnesses arising from many causes, including; dysfunctional sleep mechanisms, abnormalities in physiological functions during sleep, abnormalities of the biological clock, and sleep disturbances that are induced by external factors.

Sleep Efficiency (or Sleep Efficiency Index) - The ratio of total sleep time to time in bed. The proportion of sleep in the period potentially filled by sleep.

Sleep Episode - A total interval of sleep that may be voluntary or involuntary.

Sleep Extension - Extending sleep time by increasing the time in bed. Extended sleep alleviates the chronic sleep debt and provides an accompanying increase in alertness. It is common for many people to extend, or "catch up" on their sleep debt by sleeping for longer periods on the weekend.

Sleep Fragmentation - Brief arousals that occur throughout the night, that reduce the total amount of time spent in the deeper levels of sleep. Frequent episodes of sleep fragmentation lead to sleep deprivation.

Sleep Hygiene - Behavioral activities that either contribute to or detract from restorative sleep. Good sleep hygiene would include activities such as going to bed the same time each night, restricting caffeine intake, and avoiding napping during the day.

Sleep Hyperhydrosis - Excessive sweating during sleep.

Sleeping Pills (hypnotics) - Compounds that have a sedative effect are often used to produce sleepiness. If taken frequently, development of tolerance to the medication is common.

Sleep Inertia - Feelings of grogginess and sleepiness that persist up to 20 minutes after waking up. This results in poor performance upon awakening.

Sleep Latency - The length of time it takes to go from full wakefulness to the moment of sleep.

Sleep Log (Diary) - A daily, written record of a person's sleep-wake pattern containing such information as time of retiring and arising, time in bed, estimated total sleep time, number and duration of sleep interruptions, quality of sleep, daytime naps, use of medications or caffeine beverages, and nature of waking activities.

Sleep Mentation - All thoughts, feelings, images, perceptions, hallucinations, and active dreams that take place during sleep.

Sleep Onset - The time from when a person attempts to fall asleep until the onset of sleep. This sleep onset normally leads to NREM stage 1 sleep, but in certain conditions, such as infancy and narcolepsy, may lead to REM stage sleep.

Sleep Onset Imagery - A kaleidoscope of images and experiences during the moments following the transition from wake to sleep.

Sleep Onset REM Period (SOREMP) - REM periods within 15 minutes of sleep onset, considered to confirm the diagnosis of narcolepsy. As a result, narcoleptics often experience Hypnagogic Hallucinations as they are falling asleep.

Sleep Paralysis - Sleep paralysis is a common part of REM sleep itself but is a disorder when it strikes outside REM sleep. Usually, people with sleep paralysis are unable to perform voluntary movements either right before they go to sleep or upon waking in the morning. One of the symptoms of narcolepsy, but also experienced by some non-narcoleptic individuals.

Sleep Period - The time it takes to complete one cycle of sleep activity.

Sleep Related Accidents - Accidents caused by individuals who were sleep deprived and had impaired judgment.

Sleep Restriction - A limitation of the number of hours in bed.

Sleep Spindle - A synchronized, rhythmic EEG waveform, with a duration of about ½ second and a frequency of 12 to 14 Hz. Sleep spindles are observed most frequently during stage 2 sleep.

Sleep Stage Demarcation - The significant polysomnographic characteristics that distinguish the boundaries of the sleep stages.

Sleep Stages - Distinctive stages of sleep as demonstrated by EEG patterns. See Stages 1-4; REM Sleep.

Sleep Stage 1 (NREM Stage 1) - A stage of NREM sleep that ensues directly from the awake state. Its criteria consist of a low-voltage EEG tracing with well defined alpha activity and theta frequencies in the 3 to 7 cps range, occasional vertex spikes, and slow rolling eye movements (SEMs), and includes the absence of sleep spindles, K complexes, and REMS. Stage 1 normally represents 4-5% of total sleep.

Sleep Stage 2 (NREM Stage 2) - A stage of NREM sleep characterized by the advent of sleep spindles and K complexes against a relatively low-voltage, mixed frequency EEG background. High voltage delta waves may comprise up to 20% of stage 2 epochs. Stage 2 usually accounts for 45-55% of total sleep time.

Sleep Stage 3 (NREM Stage 3) - A stage of NREM sleep defined by at least 20% and not more than 50% of the period consisting of EEG waves of 2 cps or slower, with amplitudes of more than 75 mV (high-amplitude delta waves). It constitutes "deep" NREM sleep, or, so-called slow wave sleep (SWS), and is often combined with stage 4 NREM sleep because of the lack of documented physiological differences between the two. It normally appears usually only in the first third of the sleep episode, and usually compromises 4-6% of total sleep time.

Sleep Stage 4 (NREM Stage 4) - All statements concerning NREM sleep stage 3 apply to stage 4 except that high-voltage, slow EEG waves cover 50% or more of the record. NREM sleep stage 4 usually represents 12-15% of total sleep time. Sleepwalking, sleep terrors, and sleep-related enuresis episodes generally start in stage 4 or during arousals from this stage.

Sleep Starts - See Hypnic jerks.

Sleep talking - Talking in sleep that usually occurs in the course of transitory arousals from NREM sleep. Can occur during REM sleep, at which time it represents a motor breakthrough of dream speech. Full consciousness is not achieved and no memory of the event remains. Sleep talking probably carries no psychological or psychiatric significance, and the content should be taken very lightly.

Sleep walking (Somnambulism) - Arising from bed during a period when there is a simultaneous occurrence of incomplete wakefulness and NREM sleep. The eyes are usually open, but appear not to be focusing. If victims do not awaken during the episode, they do not remember the event. An extremely common phenomenon, occurring in up to 40 percent of children, with a peak incidence at 12 years of age. Sleepwalking typically occurs in the first third of the night during deep NREM sleep (stages 3 and 4).

Sleep-Wake Cycle - The clock hour relationships of the major sleep and wake episodes in the 24-hour cycle. See Phase Transition, Circadian Rhythm.

Sleep-Wake-Junk - Clusters of apnea and hypopnea events concentrated mainly around periods of sleep-wake transition. In general Sleep-wake-junk (SWJ) is not regarded as "real" since they do not occur during periods of sleep, but may be during semi-awake periods or "dozing". Example of SWJ

Sleep-Wake Shift (Change, Reversal) - When sleep as a whole or in part is moved to a time of customary waking activity, and wakefulness is moved to the time of the major sleep episode. This is common in jet lag and shift work.

Sleep-Wake Transition Disorder - Parasomnia disorders that occur during the transition from wakefulness to sleep or from one sleep stage to another. These disorders include rhythmic movement disorders, hypnic jerks (sleep starts), sleep talking, and nocturnal leg cramps.

Slow Eye Movements (SEM) - Slow, rolling movements of the eyes under closed eyelids that occur as one drifts to sleep in stage 1.

Slow Wave Sleep (SWS) - Stage of sleep in which EEG delta waves are present. Synonymous with sleep stages 3 and 4.

SmartPAP (Smart CPAP) - (Smart [Continuous] Positive Airway Pressure) A medical device used in the treatment of obstructive sleep apnea that provides a range of preset levels of continuous airflow, and automatically adjusts to keep the breathing passages open by sensing changes in airway integrity. The air flows from the device through a tube that connects to a nose or facemask.

Snoring - Sounds made during sleep caused by breathing vibrations in the pharynx. In the diagnosis of obstructive sleep apnea, snoring volume and frequency of occurrence often correlate with the severity of the condition. Snoring noise is recorded in both diagnostic sleep studies and CPAP titration studies.

Snoring Noise - This is recorded in both diagnostic sleep studies and titration studies, and often correlates with the severity of airway obstruction and apnea.

Soft Palate - The membranous and muscular fold on the roof of the mouth that extends back from the hard palate and partially separates the oral cavity from the pharynx.

Somatic Complaints - Awareness of pain or problems in the body.

Somnambulism - See Sleep Walking

Somnifacient - Inducing sleep; hypnotic, as in a drug.

Somnolence - Oppressive drowsiness or inclination to sleep.

Somnoplasty - The commercial name for radiofrequency treatment of snoring, nasal obstruction and some cases of mild sleep apnea. (see Radiofrequency Procedure).

Spontaneous Breathing - Spontaneous breathing can occur without ventilatory support (unassisted spontaneous breathing) or be integrated with mechanical ventilation with assisted ventilation modes (assisted spontaneous breathing). If a person is breathing "spontaneously" they are taking a normal voluntary or autonomic breath. In bilevel therapy an assisted spontaneous breath is one initiated by the patient but assisted by "pressure support". Individuals with central apnea may not spontaneously initiate a breath, and a bilevel machine with a backup rate like ASV or ST will trigger an assisted breath.

Soporific - Causing or tending to induce sleep, such as a hypnotic drug.

Stages of Non-REM Sleep - Four sleep stages associated with mixed frequency, mixed amplitude brain waves, an absence of rapid eye movements and the continuous tonic presence of low-level EMG activity from the neck and jaw muscles.

Stage 1 (NREM Stage 1) - The lightest phase of NREM sleep that directly follows the awake state. It is a transition between wakefulness and sleep, and an individual is easily aroused from this stage. It is characterized by mixed frequency, low-voltage EEG patterns with bursts of alpha activity and theta frequencies in the 3 to 7 cps range, occasional vertex spikes. Another feature is slow, rolling eye movements (SEMs). There is an absence of sleep spindles, K complexes, and REMS. In adults, stage 1 normally represents 4-5% of total sleep. However, in some sleep disorders, such as sleep apnea, this percentage of stage 1 increases. A person with sleep apnea must wake up to breath, and stage 1 follows every arousal.

Stage 2 (NREM Stage 2) - Stage of NREM sleep characterized by the advent of sleep spindles and K complexes against a low-voltage, mixed frequency EEG background similar to stage 1. Stage 2 sleep is usually free of slow rolling eye movements. A more intense stimulus is required to wake someone from stage 2 than from stage 1. In adults, stage 2 typically accounts for 45-55% of total sleep time.

Stage 3 (NREM Stage 3, Delta Sleep, Slow Wave Sleep) - Stage 3, along with stage 4 sleep is often called Delta Sleep or Slow Wave sleep. The Stage 3 EEG record consists of 20% to 50% delta EEG waves. These delta waves have a slow frequency of 2 cps or slower, and amplitudes of more than 75 mV (high amplitude delta waves). Stage 3 sleep is deeper than stages 1 or 2; it takes a more intense stimulus to awaken someone in this sleep state. Stage 3 primarily appears in the first half of the sleep episode in adults.

Stage 4 (NREM Stage 4, Delta Sleep, Slow Wave Sleep) - Stage 4 has similar attributes to stage 3, except there are a greater proportion of EEG delta waves. These high-voltage, slow EEG waves make up 50% or more of the record. This is the deepest stage of sleep; a more intense stimulus is needed to wake someone from stage 4 sleep than any other stage of NREM sleep. Young children have more stage 4 sleep and are much more difficult to arouse than adults. Both stages 3 and 4 decline with age and many older people report that their sleep is lighter than when they were young. Children typically spend 20-25% of sleep in stages 3 and 4 slow wave sleep; this decreases to less than 10% by age 60. Sleepwalking, sleep terrors, and sleep-related enuresis episodes generally occur in stages 3 or 4, or during arousals from this stage.

Stanford Sleepiness Scale (SSS) - A self-rating scale developed to quantify subjective levels of sleepiness. It consists of seven numbered statements.

Subjective Sleepiness - Feelings of sleepiness.

Substance Abuse - The excessive use of alcohol or drugs. These substances can cause sleep disturbances.

Sudden Infant Death Syndrome (SIDS, Crib Death) - The sudden and unexpected death of an apparently healthy infant, whose death remains unexplained after an adequate postmortem investigation. Death usually occurs during sleep. SIDS is the leading cause of death in infants in their first year of life, with the peak occurrence at 2 - 4 months. Although the majority of SIDS victims do not have identifiable risk factors, groups of infants at a somewhat higher risk include siblings of SIDS victims and infants born to substance-abusing mothers.

Synchronization - Two or more biological rhythms that have a 1:1 relation and maintain the same phase relationship. An example is the timing of sleep onset with respect to body temperature; sleep onset tends to occur at the same time of each temperature cycle, usually close to the minimum temperature.

Synchrony - Scheduling sleep to synchronize with the biological clock.


Tachycardia - Rapid heart rate. In adults, usually defined by a heart rate of over 100 beats per minute (bpm).

Thermocouples - Small devices placed near the nostrils to measure airflow by sensing temperature changes; expired air is warmer than inspired air.

Thermoregulation - The regulation of body temperature in mammals.

Theta Activity - EEG brain wave activity with a frequency of 4-8 Hz. It is most prominent over the central and temporal cortex.

Thoracic Excursion - Thoracic (chest) movement, which indicates respiratory effort. This is recorded in diagnostic sleep studies, and is typically measured by the placement of a sensor band around the chest. The sensor band records chest wall movement associated with respiration. This measurement is used in differentiating the type of sleep apnea.

Tidal Volume - The amount of air that is inspired or expired in an ordinary breath. Usually expressed in milliliters per second (mL/sec). Used with the respiratory rate to calculate Minute Vent. (Minute Ventilation in L/min = (Tidal volume in mL/1000) * (Respiratory rate in breaths per minute)

Titration - A method of determining optimal CPAP (continuous positive airway pressure) treatment parameters for sleep apnea. This involves automatic or manual adjustment of the CPAP device to a level that eliminates airway blockage.

Transient Insomnia - Difficulty sleeping for only a few nights..

Tolerance - In pharmacology, refers to the reduced responsiveness to a drug's action as the result of previous continued use.

Tonic (Event/Activity) - Physiological events that are constant or continuous. In sleep, atonia refers to the lack of muscle tone during REM sleep.

Tonsils - A pair of prominent masses of lymphoid tissue which are located opposite each other in the throat between the anterior and posterior pillars of the fauces (the narrow passage from the mouth to the pharynx situated between the soft palate and the base of the tongue). Composed of lymph follicles grouped around one or more deep crypts. Also called the palatine tonsil.

Tonsillectomy - Surgical removal of the tonsils.

Toothgrinding (Bruxism) - Grinding one's teeth while asleep. This occurs at some time in approximately 70% of people; most have no noticeable side effects. However, 5% of victims develop symptoms such as tooth wear, jaw pain and headaches. Episodes of grinding are more severe after stressful days.

Total Recording Time - The duration of time from the beginning of a sleep recording ("lights out") to final awakening. In addition to total sleep time, it is comprised of the time taken up by wakeful periods.

Total Sleep Time - The amount of actual sleep time in a sleep episode. This is equal to the total sleep episode less the awake time. Total sleep time is the total of all REM and NREM sleep in a sleep episode.

Tracheotomy - Surgical procedure to create an opening in the trachea (windpipe) through which breathing can occur. Occasionally used to treat severe obstructive sleep apnea that is non-responsive to other treatment.

Tracheostomy - Refers to the opening in the trachea. As a treatment for severe obstructive sleep apnea, this opening bypasses an obstruction in the airway.

Transducer - A device designed to convert energy from one form to another.

Transient Arousals - Brief awakenings from sleep.

Transient Insomnia - Difficulty sleeping for only a few nights.

Tricyclic Antidepressants (Antidepressants) - A class of medications most commonly used in the treatment of depression. Tricyclic refers to the three-ring chemical structure. Most tricyclic antidepressants also reduce REM sleep. In sleep disorders, they are used to control cataplexy, hypnagogic hallucinations, and sleep paralysis.

Trigger - In bilevel therapy, trigger refers to the switching from EPAP to IPAP at the beginning of inspiration. A bilevel machine is triggered to IPAP (inhalation pressure) and cycles to EPAP (expiration pressure). Trigger can be spontaneous (patient effort),Timed (automatic switch to IPAP at a set interval), or Adaptive (trigger is based on patient respiration rate in breaths per minute).

Turbinate - Small, shelf-like, cartilaginous structures covered by mucous membranes, which protrude into the nasal airway to help warm, humidify, and cleanse inhaled air on its way to the lungs.

Twilight Zone - A slang term that describes the waking state of individuals whose MSLT scores are 5 minutes or less. Such individuals are usually sleep deprived or suffer from a sleep disorder.

Twitch (Body Twitch) - A very small body movement such as a foot or finger jerk that is not usually associated with an arousal.


Unattended CPAP Titration Study - An automatic calibration test performed with a CPAP machine that measures the patient's airway pressure requirements during sleep. This is performed after determining that a patient has a sleep related breathing disorder such as OSA or Upper Airway Resistance Syndrome, and is likely to benefit from CPAP therapy. These findings are then used to prescribe the appropriate pressure the patient will use with their CPAP therapy.

Unintended Sleep Episode - A sleep episode that is not planned and may happen during an activity in which such an episode is hazardous, such as when driving a car or working with machinery.

Upper Airway - The part of the respiratory anatomy that includes the nose, nostrils, sinus passages, septum, turbinates, tongue, jaw, and palate.

Upper Airway Resistance Syndrome - Part of the spectrum of obstructive sleep-related breathing disorders in which repetitive increases in resistance to airflow in the upper airway lead to brief arousals and daytime fatigue. Usually associated with loud snoring. Apneas and hypopneas (see RDI) may be totally absent. Blood oxygen levels can be in the normal range.

Uvula - Small soft structure hanging from the bottom of the soft palate in the midline above the back of the tongue. It is composed of connective tissue and mucous membrane.

Uvulopalatopharyngoplasty (UPPP) - Surgical treatment for obstructive sleep apnea and snoring. This procedure is designed to open the airway behind the palate. The uvula, tonsils, and excess palatal tissue are removed. The incision is closed with sutures. The procedure is performed under general anesthesia and usually requires a one or two day hospital stay.

V, W, X, Y, Z



Wake Time - The total time scored as wakefulness in a polysomnogram occurring between "lights out" and final awakening.

White Noise - A mixture of sound waves extending over a wide frequency range that may be used to mask unwanted noise that interferes with sleep. Also called white sound.

Wilkinson Addition Test - A performance test in which the subjects add numbers for one hour. Often included in a battery of tests to measure the impact of acute or chronic sleep loss.

Withdrawal - The negative effects experienced when a person stops taking sleeping pills.




Zeitgeber - An environmental time cue that entrain biological rhythms to a specific periodicity. Known Zeitgebers are light, melatonin and physical activity. To be effective, these signals must occur when the biological clock is in a responsive phase.

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