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Most of the new science of sleep has emerged quite recently, as researchers supplement EEGs--the old-fashioned electroencephalograms that are a recording of the waves of electrical activity in the brain--with far more sophisticated imaging and neurological mapping techniques. With the new equipment, scientists are able to take increasingly detailed pictures of the sleeping brain, observing precisely what it is doing while it rests, down to the individual neuron. "In the past year or two, everything seemed to click together," says Dr. Giulio Tononi, a neurobiologist and psychiatrist at the University of Wisconsin at Madison. "Suddenly we have hypotheses that could explain lots of things. Whether they're right is a different story. But I feel different from a few years ago, when the thinking was, 'Who knows? Sleep could be anything.'"
THOSE SHIFTY EYES
Without a good theory of what sleep is for, scientists for many years concentrated on describing what it is--and treating conditions that interfere with it, such as anxiety, restless-leg syndrome and sleep apnea (see box). They've learned that most mammals, with the possible exception of dolphins and whales, cycle between two distinct phases of sleep, one of which is characterized by rapid eye movement--the famous REM sleep. The other is called, straightforwardly enough, non-REM sleep. Humans generally take about 90 minutes to complete a full cycle of REM and non-REM sleep. As dawn approaches, however, we spend more and more of that time in REM sleep and less in non-REM sleep.
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.
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.