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Unformatted text preview: and, in a departure from the Cole Porter song lyrics, even fruit flies appear to do it. Humans cer- tainly 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 re- lated questions: What is sleep, and why do we need it? An ob- vious 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 func- tion of breathing is to take in oxygen and expel carbon diox- ide. 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 in- sights for investigators to at least make reasonable proposals about the function of the somnolent state that consumes one third of our lives. What Is Sleep? U.S. SUPREME COURT JUSTICE Potter Stewart’s famous quote about obscenity—“I know it when I see it”—is a useful, if incomplete, guideline about sleep. Despite the difficulty in strictly defining sleep, an observer can usually tell when a sub- ject is sleeping: the sleeper ordinarily exhibits relative inatten- tion to the environment and is usually immobile. (Dolphins and other marine mammals swim while sleeping, however, and some birds may sleep through long migrations.) In 1953 sleep research pioneer Nathaniel Kleitman and his student Eugene Aserinsky of the University of Chicago decisively overthrew the commonly held belief that sleep was simply a ces- sation of most brain activity. They discovered that sleep was marked by periods of rapid eye movement, commonly known now as REM sleep. And its existence implied that something ac- tive occurred during sleep. All terrestrial mammals that have been examined exhibit REM sleep, which alternates with non- REM sleep, also called quiet sleep, in a regular cycle. More recently, the field has made its greatest progress in characterizing the nature of sleep at the level of nerve cells (neu- rons) in the brain. In the past 20 years, scientists have mastered techniques for guiding fine microwires (only 32 microns wide, comparable to the thinnest of human hair) into various brain regions. Such wires produce no pain once implanted and have been used in humans as well as in a wide range of laboratory animals while they went about their normal activities, includ- ing sleep. These studies showed, as might be expected, thating sleep....
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