We spend 1/3 of our lives asleep, and most people treat it as a highly enjoyable activity. We speak gleefully of “sleeping in on Sunday,” and parents of newborn infants often talk of sleep the way a man lost in the desert would speak of water. As a biological necessity, sleep appears to rank as high as the air we breathe and the food we eat, but the reasons for this need have not been fully explained. Over the past fifty years, scientists have studied the sleeping brain, and it is now clear that sleep does not reflect a general state of neural quiescence. Our eyes are shut and our limbs are limp, but our brains are working nearly full tilt. What’s going on?
It is impossible to distinguish sleep from wakefulness based solely on behavioral criteria. Who is to say a person isn’t faking that limp posture and even breathing? Sleep recording in humans, called polysomnography, is scored using information about the brain’s electrical activity with an electroencephalogram (EEG), as well as information about eye movements and muscle activity. When a person is awake, the EEG is characterized by a low voltage, fast frequency signal. As drowsiness sets in, the EEG pattern slows down and the spikes become slightly higher in amplitude. If more than 50% of an EEG has this slower rhythm, the person is considered to be in stage 1 sleep. Stage 1 is a transitional stage, and individuals aroused from stage 1 sleep usually report that they were “about to fall asleep.” A more conservative criterion for sleep onset is stage 2 sleep.
Stage 2 sleep is defined by the appearance of sleep spindles and K-complexes in the EEG. Sleep spindles, so named for their sewing-spindle appearance, are short bursts of activity thought to represent inhibitory neuronal firing. K-complexes are slow, high-amplitude burst that consist of a large negative deflection followed by a high positive deflection on the EEG (giving them their characteristic “K” shape) that may represent the brain’s responses to external or internal stimuli. Sleep stages three and four are characterized by high-amplitude, slow delta waves, so these sleep periods are often referred to as “slow wave sleep.”
Over the course of the night, a person cycles down through sleep stages 1-4, then back up to near waking levels, at which point rapid eye movement (REM) sleep begins. REM sleep is an activated stage of sleep that occurs about every 90 minutes throughout the night. During REM sleep, cerebral activity increases almost to wakefulness, and there is a general increase in autonomic nervous system activity. This sleep period is also associated with dreaming. Individuals awoken from REM sleep report dreams nearly 80% of time, whereas people awoken from non REM sleep report dreams about 7% of the time. REM associated dreams are also more story-like, more visual and longer than their non-REM counterparts. As the night wears on, people spend more time in REM sleep and less time in slow wave sleep.
The amount of time people spend in each stage of sleep changes over the course of their life span. Newborns sleep about 17 hours per day, which is over double the amount recommended for adults, and their sleep cycle is quite different from that of older children and adults. Young infants do not have the characteristic 4 stage sleep cycle. They enter REM sleep immediately and spend nearly 50% of their time in REM. Around six months of age, sleep spindles and K-complexes emerge, and the slow waves become more continuous. The time spent in REM decreases to about 30%, then begins a gradual decline until it reaches adult levels of 20-25% in adolescence. The amounts of total sleep, slow wave sleep and REM sleep level off at puberty and remain stable through adulthood, decreasing only in the later years. Elderly adults tend to sleep less as a result of more frequent and prolonged awakenings during the night.
Some have suggested that human sleep helps reprogram our brains, or that it provides emotional release that maintains our mental health. However, nearly all mammals and birds sleep, and their sleep is much like ours, patterned with distinct phases. Even fish, reptiles and insects go through periods of inactivity and unresponsiveness that are similar to mammalian sleep. The fact that sleep is so preserved across the animal kingdom suggests that is serves a critical function, but there is little agreement about what this function is.
One way to assess the benefits of sleep is to examine what ills befall us when we go without it. Sleep deprivation experiments performed on humans and animals have shown that sleep is necessary for normal functioning. Without it, a person’s attention wavers, his memory falters, and he may even experience auditory or visual hallucinations. Research has also shown that we need adequate amounts of both slow wave sleep and REM sleep. If one kind of sleep is selectively denied, the deprived person will show a “rebound” the next night by devoting more time than usual to the lost type of sleep.
The idea that sleep serves some restorative function is an attractive one. Perhaps we use those 8 hours to replenish our energy and purge our brain of unnecessary clutter. But when scientists have looked to see what nervous structures might be “resting” or what chemicals might be replenished during sleep, they have found no clear answers. Researchers have recorded from single neurons during sleep and wakefulness, and they have found that the reduction in firing rate during sleep rarely exceeds 10%. So even though the body is at rest during sleep, the brain is still quite active. In fact, during dreaming, many neurons are even more active than they are during periods of relaxed wakefulness. There is some evidence that a growth-promoting hormone that boosts protein synthesis is secreted during slow-wave sleep, but the role of REM sleep is still quite puzzling. Some researchers have suggested that it helps solidify memories formed during the day. Evidence supporting this hypothesis comes from a study showing that rats deprived of REM sleep performed more poorly on a memory task than rats with no REM deprivation.
Another theory of why we sleep focuses on the periodic nature of the sleep wake-cycle. We sleep, it says, because there are circuits in our brain that oscillate in a particular rhythm. While these circuits have endogenous rhythm, meaning that a person’s sleep cycle will keep the same schedule in the absence of environmental cues, they will also respond to temporal information such as sunlight, meals and must-see TV. The presence of an endogenous sleep-wake rhythm of about 25 hours has been observed under free-running conditions in which participants were kept in isolation from cues regarding the time of day. Under normal conditions, environmental influences (especially daylight) keep us on 24-hour cycle, called a circadian rhythm because it is the length of one day. The fact that the endogenous sleep circadian rhythm is longer than 24 hours may help explain some findings regarding jet lag. Scientists have noted that it takes twice as long to adjust to time changes produced by eastward travel as compared to westward travel. When traveling east, one is shortening the day, thereby increasing the difference between the endogenous sleep-wake rhythm and the new time zone.
Sleep on the Brain
Many areas of the brain have been implicated in sleep. Early evidence of a sleep-promoting area came from studies on nuclei in the medulla, an area of the brainstem. The raphé nuclei are a cluster of serotonin producing nuclei that runs in a thin strip sown the midline of the medulla. In cats, an animal known for its capacity to sleep, lesions of the raphé nuclei produced complete insomnia lasting for 3 to 4 days. Eventually, the cats recovered about 2.5 hours of sleep per day, but they never regained their natural amount of 14.5 hours. In support for a role for the raphé nuclei in sleep, researchers also found that injections of a chemical that temporarily blocks serotonin production caused temporary insomnia in cats. However, failure to replicate this finding in other species suggests that the raphé nuclei may not be important for sleep in all organisms.
There is little doubt that the reticular formation is important for sleep. Phylogenetically one of the oldest structures in the brain, the reticular formation is part of the brain stem and contains nuclei that separately seem to control the major indices of sleep. In both humans and cats, manipulations of cholinergic neurons in the reticular formation can affect REM sleep. Increasing the amount of the neurotransmitter acetylcholine prolongs the REM stage, while decreasing acetylcholine decreases the time spent in REM. There is also a reported case where a man experienced brain damage to the cholinergic area of the reticular formation that resulted in complete loss of REM sleep. The patient did not seem to suffer adverse affects from this loss, as he went on to complete law school with no problem. Other nuclei in the reticular formation appear to control rapid eye movements, muscle relaxation and the rhythmic stimulation of the cerebral cortex that appears to underlie dreaming. Some patients who receive damage to the nucleus responsible for relaxing the core muscles during REM sleep find themselves acting out their dreams. One man reported dreaming about playing a football game and awoke to find his bedroom in shambles.
Another nucleus that appears to be important for regulating sleep is the suprachaiasmatic nucleus of the hypothalamus. The fact that daily sleep cycles persist in the absence of environmental cues suggests that there is an internal regulator responsible for maintaining circadian rhythms. In 1967, researchers discovered that lesions of the superchaiasmatic nucleus disrupted circadian cycles of eating and sleeping in rats. Since then, scientists have shown that in many animals, lesions to the suprachaiasmatic nucleus abolish the rhythm of the sleep-wake cycle but do not affect the total amount of time spent sleeping, suggesting that the suprachaiasmatic nucleus is responsible only for the timing of sleep.
Scientists have learned much about the how the brain regulates sleep, but there is still much ignorance about what the purpose of sleep really is. Perhaps it is more about resting tired muscles than about resting the mind, allowing the body to recoup a day’s lost energy. Or perhaps the brain uses those eight hours to consolidate memory, discarding unimportant events and solidifying those that matter. These questions and more have yet to be fully answered, but they are sure to keep sleep researchers awake for many nights to come.