The S-deficiency hypothesis of depression and the two-process model of sleep regulation.

In the S-deficiency hypothesis, the changes in sleep in depression are attributed to the deficiency of a sleep/waking dependent process (Process S) involved in sleep regulation. It is further proposed that the abnormally low level of Process S is normalized by prolonged waking, and that this normalization is the basis for the antidepressant effect of sleep deprivation. Various aspects of the hypothesis have been tested in a number of studies which are reviewed. Data from healthy elderly subjects and from depressives also indicate that the decay rate of Process S may be reduced in advanced age and depression. Napping may contribute to lowering the level of S at sleep onset.