Birds do it, bees do it, and, in a departure from the Cole Porter song lyrics, even fruit flies appear to do it. Humans certainly do it. The subject is not love, but sleep. Shakespeare's Macbeth said it "knits up the raveled sleave of care" and was the "balm of hurt minds, great nature's second course, chief nourisher in life's feast." Cervantes's Sancho Panza sang its praises as "the food that cures all hunger, the water that quenches all thirst, the fire that warms the cold, the cold that cools the heart ... the balancing weight that levels the shepherd with the king, and the simple with the wise."

The simple and the wise have long contemplated two related questions: What is sleep, and why do we need it? An obvious answer to the latter is that adequate sleep is necessary to stay alert and awake. That response, however, dodges the issue and is the equivalent of saying that you eat to keep from being hungry or breathe to ward off feelings of suffocation. The real function of eating is to supply nutrients, and the function of breathing is to take in oxygen and expel carbon dioxide. But we have no comparably straightforward explanation for sleep. That said, sleep research--less than a century old as a focused field of scientific inquiry--has generated enough insights for investigators to at least make reasonable proposals about the function of the somnolent state that consumes one third of our lives.

This article was originally published with the title Why We Sleep.

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