Clues to the functions of mammalian sleep

Abstract

The functions of mammalian sleep remain unclear. Most theories suggest a role for non-rapid eye movement (NREM) sleep in energy conservation and in nervous system recuperation. Theories of REM sleep have suggested a role for this state in periodic brain activation during sleep, in localized recuperative processes and in emotional regulation. Across mammals, the amount and nature of sleep are correlated with age, body size and ecological variables, such as whether the animals live in a terrestrial or an aquatic environment, their diet and the safety of their sleeping site. Sleep may be an efficient time for the completion of a number of functions, but variations in sleep expression indicate that these functions may differ across species.

Figure 1 |. Distribution of some key sleep-regulating neuronal populations plotted on a sagittal section of a rat brain.

Circles indicate ‘REM sleep off’ neurons; purple represents serotonergic neurons (located on the midline), orange represents adrenergic or noradrenergic neurons, blue represents histaminergic neurons, red represents hypocretinergic (orexinergic) neurons. Squares indicate ‘sleep on’ neurons. The green star indicates ‘REM sleep on’ neurons. The area shaded in grey is both necessary and sufficient for REM sleep generation. The area shaded in yellow is both necessary and sufficient for NREM sleep generation. In the intact animal both REM sleep and NREM sleep involve interactions between brainstem and forebrain structures. Vlpo, ventrolateral preoptic area; Mpo, median preoptic.

Figure 2 |. Sleep time in mammals.

a, Carnivores are shown in dark red; b, herbivores are in green and c, omnivores in grey. Sleep times in carnivores, omnivores and herbivores differ significantly (P < 0.0002, F test, d.f. 2,68), with carnivore sleep amounts significantly greater than those of herbivores (P < 0.001, t-test, d.f. 24, 22). Sleep amount is an inverse function of body mass over all terrestrial mammals (black line). This function accounts for approximately 25% of the interspecies variance (d) in reported sleep amounts (regression of log weight against sleep amount, R = −0.5, P < 0.0001, n = 71). Herbivores are responsible for this relation because body mass and sleep time were significantly and inversely correlated in herbivores (R = −0.77, P < 0.001, d.f. 24), but were not in carnivores (R = −0.28, d.f. 24) or omnivores (R = −0.25, d.f. 25). It is not easy to quantify sleep parameters throughout the animal kingdom and as a result all desired parameters are rarely measured. For example, arousal thresholds are rarely systematically measured as part of a phylogenetic sleep study. Evidence for homeostatic regulation of sleep (sleep rebound) is seldom sought. Most species have not been implanted with electrodes for monitoring muscle tone and other variables. Instead, estimates are often based on visual observations, with the observer forced to intuit the differences between quiet waking and sleep. Other factors such as temperature, light cycle, food and noise conditions, which all affect sleep, have often not been controlled for. The age and health of the animals observed can vary, particularly depending on whether observations are made on animals in the wild or in the zoo. Often, observations of only one or two individuals are the source of the reported sleep amount of a given species. In animals observed in the wild, the weight of the subject is often not known, and in many cases the typical adult body weight, brain weight and other anatomical and physiological parameters cannot be or have not been precisely determined. Despite these sources of noise, significant relationships between weight, sleep time and diet are apparent. Source data for this figure were mainly from ref. .

Figure 3 |. Unihemispheric slow waves in cetaceans.

Top, photos of immature beluga, adult dolphin and section of adult dolphin brain. Electroencephalogram (EEG) of adult cetaceans, represented here by the beluga, during sleep are shown. All species of cetacean so far recorded have unihemispheric slow waves^,,–. Top traces show left and right EEG activity. The spectral plots show 1–3-Hz power in the two hemispheres over a 12-hour period. The pattern in the cetaceans contrasts with the bilateral pattern of slow waves seen under normal conditions in all terrestrial mammals, represented here by the rat (bottom traces). The brain photograph is from the University of Wisconsin, Michigan State, and the National Museum of Health Comparative Mammalian Brain Collections.

Figure 4 |. Size of the neocortex does not correlate positively with daily sleep amount.

Sleep amount is not proportional to the relative size of the cerebral cortex or to the degree of encephalization, as illustrated by these two examples. Brain photographs are from the University of Wisconsin, Michigan State, and the National Museum of Health Comparative Mammalian Brain Collections.