On the origin of early REM episodes in the sleep of depressed patients: a comparison of three hypotheses.

Shortened latency of rapid eye movement (REM) sleep is a feature frequently observed in depressed patients. Three hypotheses on the origin of early REM sleep episodes propose that short REM latency is due to (1) a phase-shift of one subset of the circadian rhythms relative to other circadian rhythms, (2) a loss of inhibition of REM sleep due to a slow wave sleep deficit, or (3) a reduction in amplitude of a putative circadian arousal cycle. From an analysis of experimental data, it is concluded that the hypothesis of a reduced circadian amplitude best explains the early occurrence of REM sleep.