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Current Perspectives on the Function of Sleep

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 of sleep 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.