Spontaneous arousal index (SAI)
The Spontaneous Arousal Index is the number of spontaneous arousals (e.g. arousals not related to respiratory events, limb movements, snoring, etc) multiplied by the number of hours of sleep. An arousal is a wake or "alpha" pattern for 3 to 15 seconds. Patients are usually not aware of arousals. There are 3 types of arousals reported out on a sleep study: those attributed to respiratory events, periodic limb movements and those that are spontaneous. Spontaneous arousals have no directly attributable cause, or cannot be linked to the first 2 reasons for arousals.
There may be respiratory events during sleep that generate "spontaneous" arousals. like snores or flow limitations. Indeed, the whole Upper Airway Resistance Syndrome is based on the presence of a large number of spontaneous arousals without the presence of scorable respiratory events such as hypopneas or apneas. These are RERAs (Respiratory Effort-Related Arousals). If you take this number, RERA, and add it to the AHI, you have the Respiratory Disturbance Index (RDI). There are numerous factors that could create a scenario for the appearance of a large number of truly spontaneous arousals, such as medications that deter sleep (pseudoephedrine, caffeine, some antidepressants, too much thyroid medication, etc.), depression and narcolepsy.