Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

So if you sometimes wake up in the middle of the night or sleep seven rather than eight hours a night, relax. In fact, humans appear to be adapted to sleep less than our ape relatives, including chimpanzees. This reduction possibly evolved about two million years ago when our ancestors apparently lost many of the features that help us climb trees, which offer a safe place to sleep in the wilds of Africa. As slow, unsteady bipeds who had to sleep on the dangerous ground, we must have been easy pickings for leopards, lions, and saber-toothed tigers before we harnessed fire. Under such conditions, our vulnerable forebears might have gone extinct had they not slept lightly, minimally, and in staggered bouts so that someone in the group was always awake to raise the alarm.