Mammalian Sleep

Knowledge about sleep comes primarily from research on mammalian species, whose daily sleep quotas range from 4 to 19 hours, with rapid eye movement (REM) sleep occupying 10% to 50% of this time. Findings of REM sleep or elements of it in monotremes have filled a gap in its evolutionary history. To some, this suggests that REM was inherited from reptiles, although the absence of REM in living reptiles casts doubt on this view. The function of sleep remains controversial.1 On one hand, restorative theories hold that brain processes during sleep sustain waking behavior (e.g., visual function, learning). On the other hand, the negative correlation of sleep quotas with body size across species suggests that sleep is a state of enforced rest most urgent in species with low energy reserves. Because most of the variance in sleep quotas remains unaccounted for statistically, supplementary theories are in order. There are strikingly strong correlations of REM sleep quotas with degree of maturity at birth—that is, altricial species, born with a low percentage of adult brain weight after a short gestation period, have higher REM sleep quotas, whereas precocial species have lower quotas. Given other fetal characteristics of altricial species (e.g., lapse of thermoregulation), REM sleep may be a carryover from fetal life. Most studies on sleep have been performed in mammals. Human beings, cats, rats, and, more recently, many mouse strains have been the most frequent subjects of sleep research, but about 100 other species have also been studied. There are at least two published reports about the daily sleep of mammalian species for each one pertaining to other classes.2 This not only makes for mammal-centeredness in thinking about sleep but also affords the opportunity for extensive interspecies comparisons that can shed light on the purpose of sleep, which is still without adequate explanation. This chapter considers relevant theories in the light of available findings. Despite their relative abundance, the mammalian data represent less than 3% of roughly 4260 extant species. The belief that some mammals do not sleep (e.g., prey species, because of a need for constant vigilance; shrews, because of the need for incessant foraging) has been superseded by systematic observations. Some species, in some circumstances, may be able to postpone sleep for long periods, or sleep may simply be difficult to recognize, as in the ever-swimming, blind Indus dolphin, whose sleep occurs in periods measured in seconds as it contends with strong river currents.3