Sleep is invisible and inconsistent. Aping death, sleep in fact prevents it; at the very least, sleep deprivation leads to premature demise (and before that, failures in mood, metabolism, cognitive function). All animals sleep, and it makes sense for none of them, evolutionarily, since it leaves the sleeper defenseless to predation. Sleep is common, public, a vulnerability we all share—even as sleep also brackets the sleeper in the most impenetrable of privacies. Nothing, everyone knows, is harder to communicate than one’s dream.
And then there’s time. Sleep seems to remove us from the general tyranny of the advancing clock. When you wake, 20 minutes could have passed as easily as three hours. But sleep defines time, dividing day and night. Humans discover circadian rhythm through the urge to sleep. That urge is, of course, cyclic, endless: always more sleep to be had. But sleep measures forward progress by consolidating our sense of the past. (Steven W. Lockley and Russell G. Foster lay out the evidence for this and other facts in their briskly informative Sleep: A Very Short Introduction.) In sleep, our brains decide what to keep and discard. Without sleep, we would dissolve into overloaded confusion.
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