From The Scientist:
Why can science still not define this most basic biological process?
This may come as a shock to the uninitiated, but a conclusive definition of sleep still eludes scientists and probably will continue to do so until the function of sleep is fully established. That’s not to say science doesn’t have a working definition of sleep.
Here is the paradox: although it seems sleep is conserved across the animal kingdom, our most precise definition of the phenomenon relies on recordings of the neocortex, the least-conserved part of the vertebrate nervous system (and altogether absent in invertebrates).
While a subcortical definition of sleep across vertebrates would already be a leap forward, it would still not easily apply to the invertebrate nervous system, and hence would not provide a truly universal definition of sleep for all animals. Evolutionary conservation lends support to the hypothesis that sleep serves a fundamental physiological need, and the best way to define sleep would be to identify this need. So instead of relying on an output (EEG, behavior) or a regulatory (neuronal circuits) definition of sleep, the ultimate definition will directly answer the simple question: why do we sleep?
Sleep likely has multiple functions, but a large and growing body of evidence supports a primary role for sleep in the regulation of nervous system plasticity … .More.
See also: Why do we need less sleep than chimps
Raymond Tallis: Sleep is still a biological – and psychological – mystery
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