Invariant relationship unites REM and NonREM sleep

Establishing structural and functional links between two distinct types of sleep, rapid-eye-movement (REM) and non-REM (NREM), that alternate and form several sleep cycles per night, has posed a significant challenge. Here we demonstrate a simple invariant relationship where the product of the duration of NREM sleep episode and intensity of subsequent REM sleep episode remains constant over successive sleep cycles of normal human sleep. This Sleep Cycle Invariant (SCI), previously predicted by a quantitative model of sleep dynamics, supports the structural and functional unity of NREM and REM sleep. The significance of SCI for understanding normal sleep and sleep disorders is highlighted by alterations in REM sleep intensity and NREM sleep episode duration being a hallmark of major depression. One-Sentence Summary The duration of NREM sleep and intensity of REM sleep have an invariant relationship across normal sleep cycles of one night.