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Clues to the functions of mammalian sleep


The functions of mammalian sleep remain unclear. Most theories suggest a role for non-rapid eye movement (NREM) sleep in energy conservation and in nervous system recuperation. Theories of REM sleep have suggested a role for this state in periodic brain activation during sleep, in localized recuperative processes and in emotional regulation. Across mammals, the amount and nature of sleep are correlated with age, body size and ecological variables, such as whether the animals live in a terrestrial or an aquatic environment, their diet and the safety of their sleeping site. Sleep may be an efficient time for the completion of a number of functions, but variations in sleep expression indicate that these functions may differ across species.

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Figure 1: Distribution of some key sleep-regulating neuronal populations plotted on a sagittal section of a rat brain97.
Figure 2: Sleep time in mammals.
Figure 3: Unihemispheric slow waves in cetaceans.
Figure 4: Size of the neocortex does not correlate positively with daily sleep amount.


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Supported by NIH and NSF and DARPA. I thank O. Lyamin for the beluga and dolphin photo and the graph of beluga sleep, and A. Siegel, L. Boehmer, R. Nienhuis, A. Rechtschaffen, I. Tobler, C. Heller, S. Ridgway, J. Horne, D. McGinty, C. Amlaner and J. Lesku for very helpful comments.

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Siegel, J. Clues to the functions of mammalian sleep. Nature 437, 1264–1271 (2005).

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