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CURRENT PERSPECTIVES ON THE FUNCTION OF SLEEP ALLAN RECHTSCHAFFEN* Introduction Sleep researchers are often confronted with such reasonable questions as the following: How much sleep do we need? What is the most efficient sleep schedule? Does anything substitute for sleep? What is the most important sleep stage? Do we need less sleep when we get older? Genuine answers to these questions require some understanding of the function of sleep. How can one say which sleep is most efficient or how much sleep we need unless we know what sleep is supposed to accomplish? Sleep researchers and clinicians can retain some posture of expertise with answers like, "Get enough sleep to keep from feeling sleepy during the day." But this temporizing answer addresses the issue of relieving discomfort , not the issue of need fulfillment. Would we feel comfortable advising patients to "Eat enough to keep from feeling hungry"? More genuine answers are difficult to come by because our understanding ofsleep function is quite uncertain. This paper aims to review for biologists the ground that sleep researchers have covered in their quest for the function of sleep and to convey our own contention that the issue is not yet resolved. This overview will not permit a detailed examination of all the theories of sleep function or the full merits of any of the theories, but we will note certain features of theoretical or empirical weakness to show why these theories have not enjoyed overwhelming acceptance. Greater attention will be given to the more recently advanced theories which have not been subjected to critical review. There are several indications that sleep is indeed functionally important, i.e., that it ultimately enhances survival. * Departments of Psychiatry and Psychology, The University of Chicago Correspondence: Sleep Research Laboratory, 5743 Drexel Ave. Chicago, IL 60637. This research has been supported by Grants MH4151 and MH 18428 from the National Institute of Mental Health.© 1998 by The University of Chicago. All rights reserved. 0031-5082/98/41034037$01.00 Perspectives in Biology and Medicine, 41, 3 ¦ Spring 1998 | 359 1.Sleep is ubiquitous among mammals, birds, and reptiles. The occasional , rare reports of sleepless species or individuals probably result from insensitive or insufficiently prolonged observation. Sleep may also be ubiquitous among lower forms, e.g., amphibians, fish, and invertebrates, but this is uncertain because they do not show the characteristic electroencephalographic sleep patterns of the higher orders [I]. Their quiescent states might be simple reductions of activity characteristic of rest rather than sleep, which is much more organizationally complex. 2.Sleep has persisted in evolution even though it is apparently maladaptive with respect to other functions. While we sleep we do not procreate, protect or nurture the young, gather food, earn money, write papers, etc. It is against the logic of natural selection to sacrifice such important activities unless sleep serves equally or more important functions. 3.Accommodations are made to permit sleep in different environments and life styles. For example, the maintenance of critical muscle tone permits perching birds to sleep [2]. Some marine mammals sleep with one half ofthe brain at a time, which enables the wake half to continue regulation of periodic surfacing for air [3]. Many prey species select safe sleeping sites. 4.Sleep is homeostatically regulated. Sleep deprivation is usually followed by sleep compensation. The rebound sleep almost never compensates completely for lost sleep time, but it may be more "intense" than normal sleep. The urge to sleep following sleep deprivation may be so overpowering as to be life threatening, as is all too frequently seen in driving accidents. Selective deprivation of specific sleep stages is often followed by selective rebounds of the lost stage, suggesting that the different sleep stages serve at least partially different functions. 5.Serious physiological changes result from prolonged sleep deprivation of animals. Rats die after about two to three weeks of total sleep deprivation —not much longer than the average of 16 days of total food deprivation [4]. Rats also die after about five weeks of selective deprivation of rapid eye movement (REM) sleep [5], which usually comprises only about 7 to 10 percent of the adult rat's existence. Given the apparent importance of sleep and...


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