What Is REM Sleep?

Abstract

For many decades, sleep researchers have sought to determine which species 'have' rapid eye movement (REM) sleep. In doing so, they relied predominantly on a template derived from the expression of REM sleep in the adults of a small number of mammalian species. Here, we argue for a different approach that focuses less on a binary decision about haves and have nots, and more on the diverse expression of REM sleep components over development and across species. By focusing on the components of REM sleep and discouraging continued reliance on a restricted template, we aim to promote a richer and more biologically grounded developmental-comparative approach that spans behavioral, physiological, neural, and ecological domains.

Figure 1.. Coalescence of REM sleep components over development in infant rats.

In early postnatal development in rats, the various components of REM (active) sleep emerge in succession, coalescing with others over time to increase the resemblance to the more familiar state of REM sleep in adults. Figure adapted and updated from [67]. See text for details. P = postnatal day.

Figure 2.. Animals that challenge traditional definitions of REM sleep.

The platypus exhibits a single mixed sleep state consisting of bill twitching and rapid eye movements (typical of REM sleep) even as the cortical EEG exhibits slow waves (typical of non-REM sleep). Rather than exhibiting atonia across all skeletal muscle groups—traditionally a defining feature of REM sleep—geese and other birds often sleep with their head unsupported and show only partial reductions in neck muscle tone, which causes the head to drop during REM sleep, but they sustain the muscle tone required to balance on one foot. Owls engage in REM sleep typical of most birds but lack rapid eye movements because their eyes are largely immobile within the skull (inset). Armadillos exhibit two sleep states that readily conform to traditional definitions of non-REM and REM sleep, except that penile erections (which are tightly coupled with REM sleep in humans and rats) occur during non-REM sleep. In contrast to the wake-like brain activity that traditionally defines REM sleep, sleeping tegus exhibit a novel 15-Hz brain oscillation, associated with rapid eye movements, that is not present during wakefulness. Also, as an ectotherm, they cannot suspend thermoregulatory effectors (e.g., sweating, shivering) like endothermic mammals and birds do during REM sleep.