Introduction

In the last few decades, scientists have made extraordinary progress characterizing the neurobiology [1], regulatory mechanisms [2], and genetic underpinnings of mammalian sleep [3]. Yet despite these great advances, the most fundamental question about sleep is still unanswered. Why animals sleep remains one of Nature's greatest unsolved mysteries. There are several reasons why this is more than just an embarrassing omission in the field. Far from being a trivial behavior, sleep with a few possible exceptions [4] is abundant in the animal kingdom [5] and comprises about one-third of human existence [6]. When animals sleep, they do so at the expense of clearly adaptive behaviors (eating, mating, defending territory) and put themselves at risk for predation. In humans, sleep is characterized by profound changes in somatic and neural physiology and its disruption is associated with physical and cognitive deficits and disease [7,8,9,10,11]. Therefore, our continued ignorance about sleep function presents a major impediment to our understanding of physiology, neurobiology, animal behavior, and sleep medicine.

In this chapter, I discuss the current status of this central problem in the field of sleep biology. I begin by discussing key concepts and theoretical issues that should be kept in mind when evaluating theories of sleep function. I then critique several categories of sleep function proposed over the last 50 years. As each of these categories can form the basis of an entire review, only selected studies are cited here. I conclude with a synthesis that winnows the thicket of competing ideas to those most likely to be true.

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