The REM cycle is a sleep-dependent rhythm.

Two studies, using data from fragmented sleep studies from three sleep laboratories, are reviewed. These studies indicate that the REM cycle is primarily governed by a sleep-dependent oscillator. These data, however, do not rule out the potential influence of endogenous or environmental variables as factors influencing the REM cycle. Further, acceptance of the REM cycle as a sleep-dependent rhythm does not lead to the denial of the basic rest-activity cycle (BRAC) proposed by Kleitman. It is questioned, however, whether the BRAC measured during waking is an extension of the REM cycle recorded during sleep.