As Nathaniel Kleitman had established through his sleep experiments in underground caves, our biological clock (the specific location of which has been identified as a cluster of cells located in the brain where the optic nerves intersect) determines the body’s rhythms for rising and falling body temperature, hormone secretion, and onset of drowsiness, which occurs not only at day’s end but also typically between 2 and 4 p.m. While that internal clock generally runs through its repetitive rhythms "about every twenty-five hours" even when there are no environmental stimuli, such as the sun rising or setting to cue wake and sleep cycles, it does seem to go through some adjustments at different stages in life. During adolescence, not only does the need for sleep increase from eight hours to ten or more hours nightly, but drowsiness sets in later than usual, as well, leading to the desire to sleep later in the morning--which is why teenagers suddenly can sleep until noon. And in later stages of life, sleep becomes fragmented, with even healthy elderly people typically awakening for a few seconds scores of times during the night, though they may not recall doing so because the arousals are often so brief as to be detectable only on EEG recordings. Interrupted sleep in turn leads to increased daytime sleepiness and the stereotypical image of grandpa nodding off in midsentence.
Another crucial question that was answered in the early days of dream research was whether REM sleep was exclusively a human phenomenon. Dement did initial work on the sleep cycle of cats, which had been favored subjects for brain studies since the 1930s. Not only were their brain structures similar to humans’, but their size and cost made them convenient research animals. After French neurobiologist Michel Jouvet demonstrated in 1960 that EEG patterns in sleeping cats were similar to the human REM pattern, other investigators probed how widespread the phenomenon was in the animal kingdom. Subsequent research has shown that reptiles don’t experience REM sleep, but mammals do, and so do the few species of birds that have been studied. REM sleep duration varies from as few as forty minutes a day in cattle to as many as seven hours a day in opossums. Predatory carnivores spend a greater proportion of time in all stages of sleep, and domestic cats, freed of the need to hunt for food, can spend more than two hundred minutes a day in REM. Researchers still have not come to a consensus on whether these variations are significant.
In humans, REM sleep begins in the womb and changes as we age. It has been detected as early as twenty-six weeks in the life of the fetus and appears to go on for twenty-four hours a day. Among newborns, REM accounts for about 50 percent of total sleep and declines steadily until a child reaches the age of four, when it stabilizes at the normal adult level of 20 to 25 percent of total sleep time. When we reach middle age, REM begins to decline, dropping to less than 15 percent of sleep time in our later years.
What purpose REM serves was a mystery to these pioneering scientists, but early clues were provided in a novel experiment conducted by Jouvet’s group in Lyon. Jouvet surgically disconnected the portion of the cat’s brain that normally paralyzes its muscles during REM and found that the cats, though still deeply asleep, would rise and appear to be stalking imaginary prey or attacking invisible enemies when they entered REM. Jouvet found that acting out pursuit behavior sometimes continued for as long as three minutes while the cat was fully asleep. As a result, he theorized that this stage of sleep in mature animals provided an opportunity for them to mentally rehearse behaviors essential for survival, so that the necessary neural circuitry could be maintained in peak condition even if that particular survival skill--say, defending against an enemy--wasn’t actually required on a daily basis in waking life. When deprived of REM sleep for more than three weeks, cats subsequently fell directly into REM sleep from waking and spent 60 percent of their time in that stage. Cats deprived of REM for periods of thirty to seventy days also experienced changes in waking behavior, becoming abnormally hungry, restless, and hypersexual.
From The Mind at Night, chapter 1, pages 1-16. Copyright 2004 by Andrea Rock. All rights reserved. No part of this book may be reproduced in any manner whatsoever, except for brief quotations embodied in critical articles with reviews, without written permission from the publisher, Basic Books.
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