People who suffer from delayed sleep phase syndrome are generally unable to reset their circadian rhythm by moving their bedtime and rising time earlier. In sleep phase chronotherapy, an attempt is made to move bedtime and rising time later and later each day, around the clock, until the person is sleeping on a normal schedule.
Here's an example of how chronotherapy could work over a week's course of treatment, with the patient going to sleep 3 hours later every day until the desired sleep and waketime is reached. (Shifting the sleep phase by 3 hours per day may not always be possible; shorter increments of 1–2 hours are needed in such cases.)
- Day 1: sleep 04:00 to 12:00
- Day 2: sleep 07:00 to 15:00
- Day 3: sleep 10:00 to 18:00
- Day 4: sleep 13:00 to 21:00
- Day 5: sleep 16:00 to 00:00
- Day 6: sleep 19:00 to 03:00
- Day 7 to 13: sleep 22:00 to 06:00
- Day 14 and thereafter: sleep 23:00 to 07:00
While this technique can provide temporary respite from sleep deprivation, patients may find the desired sleep and waketimes slip. The desired pattern can only be maintained by following a strictly disciplined timetable for sleeping and rising.
A modified chronotherapy is called controlled sleep deprivation with phase advance, SDPA. One stays awake one whole night and day, then goes to bed 90 minutes earlier than usual and maintains the new bedtime for a week. This process is repeated weekly until the desired bedtime is reached.
Sometimes, although extremely infrequently, "reverse" chronotherapy – i.e., gradual movements of bedtime and rising time earlier each day – has been used in treatment of patients with abnormally short circadian rhythms, in an attempt to move their bedtimes to later times of the day. Because circadian rhythms substantially shorter than 24 hours are extremely rare, this type of chronotherapy has remained largely experimental.
The safety of chronotherapy is not fully known. While chronotherapy has been successful for some, it is necessary to rigidly maintain the desired sleep/wake cycle thenceforth. Any deviation in schedule tends to allow the body clock to shift later again.