Sleep is the most common altered state of consciousness, as most people spend a substantial fraction of every day engaged in it. Researchers who study sleep have made tremendous use of a device known as the electroencephalograph (EEG), which has in a few short decades provided far more insight into what actually happens during sleep than thousands of years of common sense. The EEG is a device that takes information from a set of electrodes attached to the scalp and converts it into a graphic representation of the overall pattern of electrical activity in the brain. The depiction of activity, also known as brain waves, changes as behavior and mental processes change. The brain waves of an awake, alert person have high frequency (speed) and low amplitude (height), appearing as small, closely spaced, irregularly sized spikes on the graph.
Careful observation of the EEG recordings of many sleepers reveals that the roughly ninety-minute sleep cycle, repeated several times a night, can be divided into stages. When the sleeper’s eyes ﬁrst close and relaxation begins, his or her EEG immediately becomes more regular, showing a pattern of slower, rhythmic waves at speeds of eight to twelve cycles per second (cps). These are called alpha waves, and this initial, relaxed state is sometimes called stage 0. Upon actually falling asleep, the person enters Stage 1, which lasts about ﬁve minutes. In Stage 1, people sometimes experience vivid images and sensations, including weightless ﬂoating, or perhaps a sensation of falling that may cause the sleeper to jerk awake. These experiences are called hypnagogic hallucinations. A person in Stage 1 is very easy to wake up. stage 1 is followed by the deeper relaxation of Stage 2, which lasts about twenty minutes. The Stage 2 EEG is characterized by occasional sleep spindles: short bursts of rapid, rhythmic brainwave activity. The person can still be awakened easily, but is now clearly asleep.
For just a few minutes after Stage 2, the EEG begins to show large, slow delta waves, which characterize both the transitional Stage 3 and Stage 4, which are together known as slow-wave sleep. Together they last about thirty minutes, during which the sleeper is extremely difﬁcult to awaken. At this point a rather strange thing happens on the EEG—it shows a rapid passage back through Stages 3 and 2, and into a phase that looks, on the graph, almost exactly like being fully awake. This is Rapid Eye Movement (REM) sleep, named for the momentary bursts of eye activity that occur behind closed lids during this stage. Like the EEG, which shows levels of brain activity comparable to those of a fully awake and alert individual, the heart rate and breathing of a person in REM sleep return nearly to waking levels. Another unusual feature of REM sleep is genital arousal. Males typically have erections during REM sleep, and they typically outlast REM periods, taking from thirty to forty-ﬁve minutes to subside. Less is known about female genital arousal in this stage, as the phenomenon is much more easily measured in men.
The high levels of brain activity signal the most important feature of REM sleep; this is when dreams occur. Unlike the rapidly fading images of Stage 1 sleep, REM dreams are emotional and story-like, involving the same cognitive processes that operate while awake. This is why dreams often incorporate both old memories and current worries. Thus, another feature of REM sleep is especially important: sleep paralysis. During REM sleep, messages from the cortex are stopped at the brainstem, so that all skeletal muscles are relaxed and the body cannot act on anything that occurs in the dream. Even snoring usually occurs only in the other stages of sleep, stopping when REM begins. Contrary to popular belief, the eye movements are not related to dream content.
Although the full sleep cycle repeats itself, it changes somewhat over time. As the night wears on, Stage 4 sleep becomes briefer, while the REM sleep period becomes longer. In an average night, 20 to 25 percent of our sleep is REM sleep, and in infancy it is about 50 percent. Furthermore, when people are deliberately deprived of REM sleep in a laboratory setting, they experience much more REM sleep than usual, and enter the REM stage much faster, as soon as they are allowed to sleep uninterrupted. This is known as REM rebound. Clearly REM sleep must be important, but opinions and theories differ as to why.
Freud, as is well known, focused on the role of dreams in allowing the expression of the otherwise unacceptable thoughts and impulses in the unconscious mind; this has largely given way to other theories. REM sleep may play a major role in memory consolidation, for example. REM sleep deprivation reduces laboratory subjects’ ability to remember material learned just before falling asleep, and research also reveals that REM sleep increases following intense learning periods. The large amount of REM sleep observed in babies suggests that REM sleep also serves an important physiological function in brain development. Psychologists have long known that animals raised in impoverished environments, with less sensory stimulation, have less well-developed neural pathways in the brain. Perhaps the periodic high levels of brain stimulation in REM sleep occur to further develop and preserve those neural pathways. This would explain both the larger amount of REM sleep in infants, whose brains are rapidly developing and growing, and the memory effects on REM-deprived adults, whose brains haven’t had the chance to stimulate and strengthen the connections formed by the previous day’s learning.
As for the content of the dreams themselves, the most widely favored current account is known as the activation-synthesis theory. According to the activation-synthesis model, the brainstem increases the overall arousal level of the brain to waking levels during REM sleep, but this neural activation is generalized and fairly random. This spreading activation stimulates various areas of the cortex to produce what would be, in a waking state, hallucinations, along with stimulation of various areas involved in memory and thinking. Activation also spreads to various areas of the limbic system involved in emotion. Given this widespread, random, and ultimately meaningless activity, the brain’s cognitive mechanisms do what they usually do with meaningless stimuli—try to make sense of it. The activation-synthesis theory simply suggests that dreams result from the brain’s interpretation of its own random activity.
In addition to REM rebound, overall sleep deprivation also has clearly negative effects. Extensive research shows that regular loss of an hour or two of sleep nightly will accumulate over time (this is known as a “sleep debt”), resulting in slower reaction time, difﬁculty paying attention, memory problems, and impaired decision-making and judgment. The consequences of this can be fairly serious on tasks that require quick reactions, sustained attention, and clear judgment, such as driving a car. According to some estimates, sleep deprivation is a contributing factor in 200,000 to 400,000 automobile accidents annually in the United States alone, resulting in about 1,500 deaths. One study found that in both Canada and the United States, auto accidents increase immediately after the spring time change, losing an hour to daylight savings time, and decrease signiﬁcantly on the Monday after the fall time change, when everyone gets an extra hour of sleep. Sleep deprivation is also widely recognized as a major factor in airline accidents and hospital errors. Several states have now shortened the shifts worked by hospital residents, and the FAA has restricted the number of hours a pilot may ﬂy without taking time off to sleep. It is worth noting that the Exxon Valdez oil spill, the Chernobyl and Three Mile Island nuclear accidents, and the Union Carbide chemical accident in Bhopal, India, all occurred after midnight.
One effect of sleep deprivation is subtler, but hardly less dangerous: suppression of the immune system. Speciﬁcally, sleep deprivation increases levels of pathogens that would normally be suppressed by the immune system, while decreasing the levels of immune cells that ﬁght off viral infections and cancer. This may be one reason why people who are chronically sleep-deprived tend not to live as long as people who sleep eight hours a night. Clearly, sleep, the state in which we spend nearly one-third of our lives, is of vital importance.
One possible reason for the many problems sleep deprivation causes today is the extent to which modern Western society ignores the importance of circadian rhythms, the daily sleep-wake cycles that are controlled by a combination of physiological and environmental factors, but certainly not by the force of will. In earlier times and in primitive societies, in other words, for the majority of human history, these cycles were synchronized with day-night cycles. In modern times, most of us synchronize our sleep periods with the clock instead, staying up well into the night as well as waking before sunrise. If synchronization with an external sign (the clock or the sun) was all that were involved, the process would be fairly easy and problem-free. Unfortunately, we also have an internal, biological clock to contend with, which evolved over hundreds of thousands of years of human experience before alarm clocks, graveyard shifts, and jet lag came along. When people are removed from the inﬂuence of external cues, perhaps by being placed in a windowless laboratory with no clocks or watches for several weeks, they continue to follow a steady sleep-wake cycle, but oddly enough, it is a twenty-ﬁve-hour cycle rather than a twenty-four-hour cycle, in which they fall asleep 1 hour later each night. The external cue (the sun) is used by the brain to reset its clock each day, and in its absence, the clock gets further and further off.
How this works is fairly straightforward: circadian rhythms are largely governed by blood levels of the hormone melatonin, secreted by the pineal gland. The pineal gland is located near the optic chiasm, the place where the optic nerves, bringing sensory information from the eyes, cross over one another, and it receives information about light levels from those nerves. When light levels indicate that the time to sleep is approaching, more melatonin is secreted, and when daylight returns, melatonin levels drop. In the absence of external light cues, this system cannot function properly. Nor can it function properly in the presence of the wrong light cues. When people travel across several time zones in a short period of time, the fatigue of jet lag can take from a few days to several weeks to wear off, as the internal clock becomes synchronized with the new day-night cycle. The same problem occurs when people who have been working during the day are assigned to a night shift, sometimes in such high-risk areas as hospitals, airline cockpits, and factories. If people would then stay on the new schedule for several weeks, they might be able to adjust fully, but research suggests that most workers never make a complete adjustment to night shifts, sleeping poorly during the day and remaining overtired and sleepy at night. The problem is due in part to these workers’ tendency to revert to a normal day-night schedule on weekends, so that they can spend time with their family and friends. Another problem is employers’ tendency to rotate people in and out of night shifts on a regular basis, so that they are switched back to a daytime schedule just as they are getting acclimated to the night schedule.
One other quirk of the average human’s circadian rhythm is ignored by modern Western societies: the daily cycle has two high points (most awake and alert) and two low points (least alert). One of these lows is experienced in the early a.m. hours by most people, when they are usually asleep anyway, but the other is twelve hours away, which puts it in the earlyto mid-afternoon for most people. Throughout Latin America, this fact is an intrinsic part of the culture, enshrined in the custom of the siesta. In the United States, however, napping is generally regarded as something small children need but as a sign of laziness in adults. This is unfortunate, since many sleep-deprived teens and adults would function much better if they had a small amount of sleep in the early afternoon. That feeling of overpowering sleepiness with which so many of us are familiar is the body’s way of sending an important message. We continue to ignore it at our peril.
- Coren, Stanley. Sleep Thieves. New York: Free Press, 1997.