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A phase advance of some circadian rhythms (e.g., body temperature and rapid eye movement [REM] sleep propensity) relative to the sleep-wake cycle is thought to be implicated in the pathophysiology and pathogenesis of some affective disorders. Since this phase disturbance can be induced in normal subjects by acutely delaying their sleep onset, it follows that the outcome of this experimental procedure should resemble the symptoms of depressive illness. This hypothesis was tested by imposing a 6-hour phase delay of sleep in 10 young male subjects. There were reliable changes in observers' ratings of mood and in some self-report measures. For the most part, the effect was modest, being largely confined to variations within normal limits. Two of the subjects, however, were noticeably depressed when interviewed after either the first or the second night of phase shift.