Why We Sleep

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If you look at the EEGs of people in REM sleep, you see a pattern that shows lots of brain activity—and if you wake them up during it, they will tell you that they have just been dreaming. Any dreams in non-REM sleep usually consist of no more than a simple image or two.

But despite all the mythology that surrounds dream imagery, scientists who have searched for the hidden purpose in dreams haven't had much luck. The consensus among sleep researchers today is that dreams are nothing more than random recycling of bits and pieces of the previous day's events.

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EEGs taken during non-REM sleep reveal four distinct stages as we progress from light to very deep sleep. Stages 3 and 4 of non-REM sleep are characterized by distinctive low-frequency electrical waves; researchers call that slow-wave sleep. Intriguingly, humans spend much more time in slow-wave sleep during the first three hours of the night than they do in the hours just before waking. Children are champion slow-wave sleepers, which is why they sleep so soundly when being carried from the car to bed. Adults, on the other hand, get less and less slow-wave sleep as they age, which may be one of the reasons they wake up more often in the night.

For years sleep researchers focused most of their attention on REM sleep because, frankly, it seemed more interesting—all those dreams and everything. But they kept running into blank walls. Early work that tried to link REM sleep to learning foundered when scientists discovered that their test subjects could remember long lists of new words or facts whether or not they got any REM sleep. Indeed, an Israeli man with a piece of shrapnel in his brain became famous in sleep circles for not getting any REM sleep at all. Despite that, he went to law school and seems to have no trouble handling new situations. Many investigators gave up trying to figure out what sleep was for and focused their attention on treating various sleep disorders, such as insomnia and narcolepsy.

New Tools, New Ideas
Two things happened in the mid-1990s, however, that revived research into the fundamental purposes of sleep. A 1994 study by scientists at the Weizmann Institute in Rehovot, Israel, suggested that researchers had been looking at the wrong kind of memory processing. And the technology for peering inside a sleeping brain got a whole lot better.

What the Weizmann researchers found was that your ability to recognize certain patterns on a computer screen is directly tied to the amount of REM sleep you get. Such skills depend on something called procedural memory, which is needed for any task that requires repetition and practice. Remembering a fact, like the name of the first U.S. President, is an example of declarative memory, a different kind of capability that apparently is not affected by REM sleep. Says Robert Stickgold, a cognitive neuroscientist at Harvard Medical School: "We were basically naive about memory."

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