Do all animals sleep?
Jerome M. Siegel
Department of Psychiatry, School of Medicine, University of California, Los Angeles and Neurobiology Research (151-A3), VA-GLAHS, North Hills, CA 91343, USA
Some animals never exhibit a state that meets the behavioral deﬁnition of sleep. Others suspend or greatly reduce ‘sleep’ behavior for many weeks during the postpartum period or duringseasonal migrations without any consequent ‘sleep debt.’ Rats die from one form of sleep deprivation, but sleep loss has not been shown to cause death in well-controlled studies in other vertebrate species. Some marine mammal species do not show evidence for REM sleep, and convincing evidence for this state in reptiles, ﬁsh and insects is lacking. The enormous variation in the nature of rest andsleep states across the animal kingdom and within the mammalian class has important implications for understanding the evolution and functions of sleep. Introduction An assumption made by many is that all animals sleep or that all animals with nervous systems sleep. A Google search for the phrase ‘all animals sleep’ brings up 3090 hits. A Google search for the phrase ‘do all animals sleep’ brings uponly 327 hits, and many of these answer the question in the afﬁrmative. Many neuroscientists and sleep researchers  have assumed (without, I would contend, good evidence) that all animals sleep. A further assumption is that sleep deprivation is lethal. Together, these two assumptions suggest that a universal, vital function is accomplished in sleep. Deﬁning sleep We all understand what it meansto be asleep, but it is not always obvious whether observed animals are experiencing the same state. Sleep must be distinguished from circadian changes in alertness controlled by the suprachiasmatic nucleus and other body clocks. Most animals need to adjust their activity to optimal conditions of prey availability, predator threat, sexual opportunities, temperature and other variables affectingsurvival that vary with time of day. Hence, even when completely sleep deprived, most animals exhibit a marked circadian rhythm of alertness and activity, and reduced responsiveness and inactivity. The presence of such periods of reduced activity and alertness cannot be assumed to be sleep. Sleep persists in animals in which circadian rhythms have been eliminated . It is the summation of thecircadian and homeostatically regulated sleep processes that determines our alertness . Sleep must be distinguished from hibernation and torpor, states that have distinct physiological correlates. It is also very important to distinguish sleep
Corresponding author: Siegel, J.M. (firstname.lastname@example.org).
from rest, a state of reduced activity without loss of consciousness or greatly reducedresponsiveness. Sleep is generally deﬁned as a rapidly reversible state of immobility and greatly reduced sensory responsiveness. An important further criterion is that sleep is homeostatically regulated, namely that lost sleep is made up with an increased drive for sleep and a consequent ‘sleep rebound.’ It would appear to be highly maladaptive for animals to be driven to make up for lost sleep at a timeof danger and stress unless some vital function was being subserved. Indeed, it has often been asked why some animals have not evolved a quiet waking state as a substitute for sleep. Surely it would be more adaptive to reduce activity and maintain vigilance. As we shall see, a survey of the literature reveals that evolution might well have produced species that have states that can be betterdescribed as quiet waking than as sleep. Two types of sleep have been identiﬁed in mammals, non-REM sleep (slow-wave sleep) and REM sleep. At the neuronal level, non-REM sleep is characterized by greatly reduced activity in brainstem systems [4,5]. Forebrain neuronal activity rates are reduced below those of quiet waking, although the predominant change is from irregular discharge patterns to a...