Studies with Drosophila melanogaster, however, demonstrated that flies, also, become less responsive, i.e., sleep, when they remain quiescent for a few minutes. Scientists have been hesitant to attribute sleep to reptiles, amphibians, fish, and especially invertebrates, preferring the noncommittal term “rest” in the absence of electrophysiological signs resembling those of mammals and birds. Most studies found signs of sleep, both behavioral (quiescence and hyporesponsivity) and electrophysiological (e.g., the slow waves of non-rapid eye movement sleep). Only a small number of species-mostly mammals and birds-have been evaluated in detail with respect to sleep. The decreased ability to react to stimuli distinguishes sleep from quiet wakefulness, while its reversibility distinguishes sleep from coma. Sleep is a reversible condition of reduced responsiveness usually associated with immobility. But how strong is the evidence supporting it? And are there counterexamples?Ĭorollary 1: Are There Animals That Do Not Sleep? This “null hypothesis” would explain why nobody has yet identified a core function of sleep. ![]() Also, if sleep is just one out of a repertoire of available behaviors that is useful without being essential, it is easier to explain why sleep duration varies so much across species. ![]() But what if the search for an essential function of sleep is misguided? What if sleep is not required but rather a kind of extreme indolence that animals indulge in when they have no more pressing needs, such as eating or reproducing? In many circumstances sleeping may be a less dangerous choice than roaming around, wasting energy and exposing oneself to predators. Everybody knows that sleep is important, yet the function of sleep seems like the mythological phoenix: “Che vi sia ciascun lo dice, dove sia nessun lo sa” (“that there is one they all say, where it may be no one knows,” Wolfgang Amadeus Mozart and Lorenzo da Ponte, Così fan tutte).
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