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….