Rapid eye movement sleep cycle, clock time and sleep onset.

The phase of the REM sleep rhythm was studied in 10 normal subjects each of whom was sleep studied for 4 consecutive nights. For analysis, each night of sleep was aligned according to clock time and each minute was scored as REM or non-REM. With these data, REM probability was found as a function of clock time. Fractional harmonic analysis indicates a 90 min periodicity. The REM probability curve shows peaks occurring at 1:30 a.m., 3:15 a.m., 4:30 a.m., 5:45 a.m. and 7:00 a.m. Statistical measures comparing the time of REM sleep across subjects suggests that subjects tend to have REM sleep at the same time of the night as each other. The influence of elapsed time after sleep onset on REM sleep is also reestablished. Results indicate that the time of REM sleep is determined by both clock time and time of sleep onset, suggesting two clocks, one sleep dependent and the other related to the basic rest activity cycle (BRAC), which are responsible for driving REM sleep. Furthermore, the similarity of REM times across subjects indicates the possible existence of an extra-personae REM driving force linked to clock time and possibly the BRAC.