REM Sleep as a Biological Rhythm

Abstract

Biological rhythms are processes characterized by recurrence over a particular time period, with the period of recurrence often referred to as τ. The inverse of the period of a rhythm is its frequency, a measure less often used. Circadian rhythms, for example, refer to processes with a period of about a day, and ultradian rhythms are those with periods shorter than a day. REM sleep has a period of about 90 min in the adult human, about 22 min in the cat, and about 12 min in the rat. This feature of alteration of ultradian REM sleep period with body and brain size is not seen in circadian rhythms, which maintain an approximately 24-hr period across all species, regardless of size. This constancy of circadian period likely reflects function: circadian rhythms prepare behavior and physiology for events in the external world that are clocked by the circadian period of the earth’s rotation, such as sunrise or sunset. Although the functions of REM sleep remain speculative, adjusting internal events to coincide with timing of external events thus seems excluded as a function. A point relating circadian rhythms and the ultradian REM sleep rhythm is that in many species, including man, the time of occurrence and other parameters of REM sleep are modulated by circadian rhythms, although the occurrence of REM sleep is not dependent on the presence of either a circadian rhythm or an intact circadian oscillator.