Delayed sleep phase syndrome has been linked to psychiatric mood disorders, such as time- or event-specific depression (e.g., seasonal affective disorder; SAD). Like other complications of DSPS, depression may cause, as well as result from, the disorder. Some type of sleep disorder is present in approximately 90 percent of people who suffer from depression. In psychiatric patients, improvement in sleep may indicate that recovery is imminent.
Rapid eye movement (REM) sleep quality and duration may affect depression. REM sleep onset and REM-related wakefulness patterns in cases of delayed-advanced sleep phase are similar to those associated with depression. This basic premise among researchers has led them to more carefully observe the role that circadian rhythms play in depression, which, in turn, has given way to the REM Phase Advance Hypothesis (Wever 1979).
In summary, researchers hypothesize that there are two major circadian processes at work in sleep-mood function that regulate (1) REM sleep and temperature and (2) sleep-wake patterns. The REM process is phase-advanced relative to the process governing sleep onset. A desynchronized shift might cause REM abnormalities characteristic of depression and sleep disorders.
However, this assumption is qualified by the fact that DSPS and other circadian disorders are treated successfully by the purposeful shift of sleep-wake patterns and, ultimately, by circadian rhythm change. It stands to reason that methodical change in sleep-wake routine affects rhythmic patterns, at least temporarily. So, it seems that the delicate balance between normal rhythm and proper function is as adaptable to change as it is vulnerable.
Rapid eye movement sleep manipulation produces effects similar to those caused by antidepressant medications (e.g., tricyclics). These treatments allow patients to function better in daily routine for longer periods of time. Unfortunately, results of this research are still inconclusive.
Non-24-Hour Persistent Disturbance
Although its exact incidence and prevalence are not known, non-24-hour sleep-wake syndrome is a related non-transient sleep disorder that affects fewer people than DSPS. This condition results when a person inadvertently delays bedtime and then attempts to sleep at a conventional time. The underlying cause of the syndrome is, again, circadian rhythm disruption. The patient repetitively delays sleep, influences rhythm, and then surprises circadian patterns with a sudden re-adherence to conventional sleep indicators. Disruption stems from the inability to phase-compensate for each day that bedtime has been delayed.
In DSPS, patients usually train themselves to gradually respond to external indicators of sleep-wake time. With non-24-hour sleep-wake syndrome, the patient essentially hosts a free-for-all of stimuli and dependent circadian rhythm, in which the once predictable coupling of these two sleep agents becomes erratic and disruptive.
Often, phase advance techniques are used to treat non-24-hour sleep-wake syndrome until conventional bedtime is consistent.