The Evolution of Mammalian Sleep

Abstract

Mammalian brains and bodies tend to increase in size in evolution, requiring increasing hours of feeding per day—which is only feasible when daily sleep requirement is low. Mammalian species can sleep as little as 3 h day −1 and as much as 19 or 20 h day −1 , and typically, small species sleep longer hours than large species. Given that early mammals were very small, it can be expected that larger mammals appeared as sleep requirement evolved, decreasing with increasing body and brain mass. However, the correlation between brain (or body) mass and total daily sleep duration is not straightforward, with many examples of large mammals that sleep many hours per day, such as carnivorans. Besides, how could body mass possibly be related to brain mechanisms that control sleep duration? This chapter reviews the recent finding that daily sleep duration scales universally with the ratio between the density of neurons in the cerebral cortex and the total surface area of the cortex (D/A), which is presumed to determine the rate with which sleep-inducing metabolites accumulate in the cortical parenchyma during waking, both across adult species and in rat development. Because D/A decreases with increasing numbers of neurons, I propose that increasing numbers of cortical neurons in early mammalian evolution led to a decreased sleep requirement that allowed for more hours of feeding and increased body mass, which would then facilitate further increases in numbers of cortical neurons through a larger caloric input. Such coupling of increasing numbers of neurons to a decreased sleep requirement may thus have not only allowed but also driven the trend for increasing brain and body mass in mammalian evolution.