The recent News Focus (9 March, p. 1360) "Hunting for meaning after midnight" summarizes recent views of medically oriented sleep researchers, including their disagreements. Largely at the root of the disagreements is their equating of the ultimate function of sleep, that is, the function responsible for sleep’s origin, to neural events that occur during sleep.
Thus, each of the researchers studying neural events of sleep disagrees with most of the others. Each proposes that his or her particular event is the function of sleep. They make no distinction between whether the proximate or ultimate function is meant, but the ultimate function can be assumed, because the long-term goal of most sleep researchers is to solve the "supreme mystery" the enigma of sleep’s origin. Proposed functions include active learning, identifying rules of cause and effect, finding missing connections, consolidating memories, strengthening memories, and improving waking performance. One or another combination of such actions would comprise what the evolutionist regards as the proximate functions of sleep.
The key to elucidating sleep’s ultimate function concerns understanding why a second vigilance state became necessary to accommodate certain activities of the brain. Why couldn’t these activities, like the others, simply have been accommodated during waking? I suggest that probing the "supreme mystery" from Darwinian evolutionary perspectives would be most productive. Thus, an evolutionary analysis might lead expeditiously to sleep’s ultimate function, proceeding as follows.
The evolution of sleep made it possible, for the first time, to defer some brain activities to a second vigilance state. This new flexibility probably achieved or maintained maximal efficiency of brain operation. One would expect sleep to have evolved in animals whose waking brains could no longer efficiently support both activities critical for survival and large, increasing amounts of non-urgent activities. At that time, some non-urgent activities, of which memory processing would have been a leading candidate, would have been deferred to the new sleep state.
It remains to identify the critical activities. Animals that move very slowly mussels, sea anemones, starfish, many worms, etc., need no sleep (1). However, all fast moving animals [with rare, reconcilable exceptions (2)] need sleep. Accordingly, one critical activity of the waking brain during natural selection for sleep probably supported rapid movements. Since rapidly moving animals have complex eyes or eyespots, another critical activity during selection for sleep probably supported vision (1, 2). Accordingly, when non-urgent neural activities began to conflict with the brain’s waking processing for fast movements and vision, the conflict probably was avoided by deferring the conflicting, non-urgent activities to the new sleep state.
Accordingly, the ultimate function of sleep probably is to enable brains to operate highly efficiently at all times. During sleep, brains can engage lengthily in non-urgent functional activities without danger of conflicting with critical activities of the waking brain.
J. Lee Kavanau
University of California at Los Angeles, P.O. Box 951606, Los Angeles, CA 90095, USA.
References
1. J. L. Kavanau, Neuroscience 79, 7 (1997).
2. J. L. Kavanau, Sleep Med. Rev. 9, 141 (2005).
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