Neuroscience

More neurons mean less need for sleep

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Science  27 Nov 2015:
Vol. 350, Issue 6264, pp. 1052
DOI: 10.1126/science.350.6264.1052-a

Species with more neurons, such as this Gelada baboon, may need less sleep

PHOTO: MICHAEL NICHOLS/NATIONAL GEOGRAPHIC CREATIVE

Sleep is seemingly universal among animals. Daily sleep time varies considerably between mammalian species and also during mammalian development, yet we still don't know what drives this variation. Herculano-Houzel hypothesized that sleep-inducing metabolites produced during waking hours accumulate more slowly in brains that have a smaller density of neurons underneath a unit surface area that gets washed by cerebrospinal fluid during waking. In 24 mammalian species and several postnatal stages in the developing rat, there was indeed a correlation between the ratio of neuronal density to brain surface area and daily sleep duration. The evolutionary addition of neurons may have decreased the need for sleep, allowing a species to feed for longer, and thus facilitated further increases in neuronal numbers.

Proc. R. Soc. London Ser. B 282, 1816 (2015).

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