Thus far in this chapter, you have learned about how you consciously experience the sights and sounds of the external world. Yet, during about a third of your life, you don’t experience them. About a third of the time, if someone held a picture in front of your face, tapped you gently on the arm, or whispered in your ear, you wouldn’t even notice. These are the times when you are asleep.

The word “sleep” seems to refer to a single state. You may believe that when you fall asleep at night, your mind and body enter a resting state—the state of sleep—and that this state continues uninterrupted for about 8 hours, until you wake up. If this is how you think about sleep, you’re in for a surprise.

TWO TYPES OF SLEEP. Sleep is not a single state. You alternate between two different types of sleep, usually about every 90 minutes, throughout the night (Siegel, 2003). One is called REM (rapid eye movement) sleep because, during this sleep phase, your eyes move rapidly back and forth. The other state is non-REM (sometimes “NREM”) sleep.

Bodily states during REM and non-REM sleep differ markedly (National Institutes of Health [NIH], 2011; Biological Sciences Curriculum Study [BSCS], 2003):

Brain functioning also differs in REM versus non-REM sleep. Unlike what happens when you are awake, adjacent neurons in the cortex fire at the same time during non-REM sleep. This synchronous activity produces distinctive brain waves that can be detected by EEG methods (see Research Toolkit). But during REM sleep, brain activity is different; it resembles brain activity during waking periods more than activity during non-REM sleep. In REM sleep, (1) brain cells in the cortex and brain stem are as active as during waking states, (2) the brain consumes as much energy as when you are awake, and (3) brain waves resemble those of wakefulness (Siegel, 2003).

Brain-imaging evidence reveals that REM-sleep eye movements are associated with activity in multiple brain regions (Hong et al., 2009). These include not only the visual cortex, but also nonvisual sensory areas, the motor cortex, regions of the brain that process language, and the thalamus (Hong et al., 2009). The brain’s multiple systems that produce conscious experience thus are active during REM sleep (Figure 9.5).

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People dream during both REM and non-REM sleep, but during REM sleep, dreams are more frequent and more vivid. When awakened during a REM period, people report having been in the midst of a dream more than 80% of the time, whereas during non-REM sleep periods, they report this only about one-third of the time (Stoerig, 2007). REM dreams commonly are more vivid than non-REM dreams, though people do sometimes report vivid dreams during non-REM periods (Solms, 2000).

What is the function of REM sleep’s rapid eye movements? Brain-imaging evidence suggests some possibilities (Hong et al., 2009). One is that the eye movements are “scanning” the dream imagery. When awake, your eyes move frequently as you scan the environment (see Chapter 5). Similarly, when asleep, their movement may indicate your scanning of your dream world. Consistent with this idea, when people’s eyes move more frequently, they report (when awakened) more vivid dream imagery (Hong et al., 1997). Another possibility is that the eye movements contribute to the production of dream images (Hong et al., 2009). Eye movements may be part of a mental system that retrieves, from memory, visual images that are incorporated into dreams.

With all the brain activity that occurs during REM sleep, you might wonder how you manage to stay asleep. As you learned, the motor cortex is active during REM sleep. Why, then, aren’t you running around your bedroom while you are having a dream about running, or shouting out loud whenever you are shouting in your dreams?

During REM sleep, your body remains still because your brain “switches off” the body’s main system for moving your muscles. It does this by altering brain chemistry. The brain changes the release of neurotransmitters that, during waking periods, activate cells that control motor movement (Siegel, 2003). As a result, during REM sleep, you are largely paralyzed; the muscles in your arms and legs do not move at all. (Other muscles, such as those that control the heart and eyes, function normally.) Sometimes this paralysis lasts for a brief period after people wake up, and they experience a potentially frightening state known as sleep paralysis, in which a person is awake but temporarily unable to move or talk (Hishikawa & Shimizu, 1995).

Evidence of how the brain switches off muscle movements during dreams comes from a study in which researchers disabled the brain’s switching-off mechanism. This was done in research with cats. Researchers intentionally damaged an area in the brain stem that is required to switch off muscle movement during sleep (Jouvet & Delorme, 1965). The cats fell asleep normally but, while sleeping, would walk, run, and sometimes even fight. The cats appeared to be acting out their dreams!

SLEEP STAGES. When during the night do these periods of non-REM and REM sleep occur? You might expect that it varies. Maybe different people experience non-REM and REM sleep at different times, or perhaps any given person experiences different sleep patterns from one night to the next. If this is what you expected, you are in for another surprise about sleep. Sequences of non-REM and REM sleep do not vary significantly from person to person, or from one night to the next. Rather, essentially every night, every person experiences the same sleep stages, that is, the same sequence of changes in REM and non-REM sleep (Silber et al., 2007).

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During the first 90 minutes after you fall asleep, you experience a series of three non-REM sleep stages: NREM-1, NREM-2, and NREM-3 (Silber et al., 2007; Figure 9.6). Each one, in order, is a progressively deeper stage of sleep. Your heart rate and breathing slow from one stage to the next, and your brain waves look less and less like those that occur when you are awake. But after about 90 minutes, a remarkable thing happens. Instead of staying in deep, NREM-3 sleep, you emerge from it and experience your first period of REM sleep. This first REM period is relatively brief, and after it you head back down again, through the stages of non-REM sleep. This cycle repeats itself during the night, with the only change being that, as the night progresses, the REM periods become longer and you do not descend all the way down to the deepest stage of non-REM sleep.

Biochemical and neural processes are responsible for the transitions from non-REM to REM sleep (Dement, 1978). These biological processes cannot be altered; even experimental manipulations explicitly designed to alter them are ineffective. For example, researchers have found that, by applying a small electrical impulse to a certain region of cats’ brains, they could induce REM sleep in the cats. However, if a sleeping cat had just emerged from a REM sleep cycle, this same manipulation had no effect (Dement, 1978). Once it completed one REM sleep cycle, the cat had to experience a non-REM sleep period before it could experience additional REM sleep.

Small sets of cells in lower regions of the brain control this sleep cycle. A group of neurons in the base of the forebrain known as “sleep-on” neurons induce sleep. Among the factors that activate these neurons is increased body heat (Siegel, 2003); it is not surprising, then, that people become sleepy on a hot summer afternoon. A group of “REM on” neurons in the brain stem initiates REM sleep, which is terminated by a different set of brain-stem neurons known as “REM off” cells (Solms, 2000). Discovery of these brain mechanisms again shows that the sleep cycle is a thoroughly biologically determined process.

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RESEARCH TOOLKIT

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Studying sleep scientifically is challenging. Scientists need to observe normal 8-hour nights of sleep, which people usually experience at home. Yet they need to record physiological activity during sleep, which requires equipment found only in a lab.

SLEEP LABS. Researchers meet this challenge by conducting research in sleep labs. A sleep laboratory is a scientific facility for studying sleep that includes a hotel-like room in which research participants can spend the night, as well as equipment for monitoring participants’ biological rhythms and brain activity while they are sleeping.

Sleep labs have two big advantages for research. First, researchers can monitor research participants’ heart rates, brain waves, eye movements, and other aspects of physiology while they are sleeping. Second, they can control environmental factors that would be impossible to control in the home environment. This control enables experimental tests of competing theoretical explanations of sleep, waking, and internal biological states. Here’s an example.

MANIPULATING LENGTH OF DAY. People (and other mammals) experience a circadian rhythm, an approximately 24-hour cycle of changes in internal bodily processes, including those involved in body temperature, hunger and feeding, and sleep and wakefulness. Why is the cycle 24 hours long? One possibility is biological. Internal physiological processes may produce and maintain the 24-hour cycle. A second possibility is environmental. People live in environments that shift according to 24-hour cycles: The sun sets and rises; alarm clocks ring; late-night talk shows are broadcast. These external cues, rather than internal biology, may explain the regularity of our circadian rhythms.

How could one find out which factor—inner biological or outer environmental cues—is most influential? An ideal study would create an “alternative world” in which all environmental cues, such as patterns of light and dark, were rescheduled. One could then see if shifts in the external environment change people’s biological rhythms.

Researchers have created this alternative world in sleep labs. In one study (Czeisler et al., 1999), participants lived in a sleep lab for a month. They had no access to watches or clocks. Unbeknownst to the participants, the researchers created different light–dark schedules in the lab. For some participants, darkness and daytime light repeated every 28 hours, instead of the normal 24. For others, the light–dark cycle repeated every 20 hours. Researchers recorded participants’ physiological states (e.g., body temperature) throughout, to determine whether their normal 24-hour bodily rhythms shifted when living in 20- or 28-hour days.

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Remarkably, the shifts in environmental cues had no effect on bodily rhythms. The circadian rhythms of people experiencing 20- and 28-hour days were 24.17 and 24.15 hours, respectively (Czeisler et al., 1999)—times that were essentially the same and that corresponded closely to the 24-hour period of a normal day. The body, then, has an internal clock that can maintain a 24-hour cycle of biological processes, even when environmental cues vary from this 24-hour period.

It’s possible to imagine a world in which animals did not sleep. They might rest, to save energy, yet not lapse into the states of unawareness that comprise sleep. From the perspective of any individual organism, this sleep-free world would seem advantageous. Sleep is dangerous. The sleeping animal is vulnerable to attack from one who is awake.

Yet, in the real world, all mammals sleep. In fact, all known mammals experience both non-REM and REM sleep (Capellini, Barton et al., 2008). Over the course of evolution, sleep must have possessed some advantage that outweighed its costs.

Identifying this advantage is difficult. Scientists have not reached complete agreement on the primary benefits of sleep. They have, however, identified some possibilities.

Non-REM sleep may help the body repair itself. During non-REM sleep, the body’s rate of metabolism—that is, the rate of internal chemical reactions necessary to life—is lower. Lower metabolic rate gives the body time to repair damage that may have occurred during wakefulness (Siegel, 2003). But some evidence calls into question whether bodily repair explains the need for sleep. Consider animals that hibernate. In the period right after awaking from hibernation, animals need large amounts of non-REM sleep, rather than the small amounts you might expect if sleep were required for bodily repair and the body had been repairing itself during the hibernation period (Capellini, Nunn et al., 2008).

An alternative possibility is that sleep is needed not to repair the body as a whole, but to repair only one bodily organ in particular: the brain. When animals are awake, their bodies produce biochemical waste products that can build up in the brain and harm its functioning. Changes that occur within the brain when animals sleep make it easier for the brain to remove these waste products (Xie et al., 2013).

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Other biological needs may contribute to sleep patterns. One possibility is food needs. Herbivores (animals that eat plants but not meat) need more time to forage for food than do carnivores (meat eaters). They also tend to get less REM and non-REM sleep than carnivorous species. Their need for extra foraging time may limit the time they have available to sleep (Capellini, Nunn et al., 2008). Another need involves the brain. Especially early in life, the brain must establish neural connections. The mental activity of REM sleep may aid this neural process, helping the brain to establish connections required for efficiently processing environmental stimuli when awake. In all mammalian species, REM sleep is more abundant early in life than in adulthood (Frank, 2011).

In summary, it is difficult to know exactly why animals need to sleep. But there is no question that they do need their daily shut-eye, as research on sleep deprivation illustrates.

In 1959 a radio announcer named Peter Tripp staged a “wakeathon.” While being observed by medical personnel, Tripp stayed awake for a record 201 consecutive hours. The record didn’t last for long. In 1964, a high school student named Randy Gardner broke it by staying awake for 264 hours—11 consecutive days!

SLEEP DEPRIVATION. What’s it like to go for days without sleep, or to experience extreme sleep deprivation? Being sleep deprived does more than just make you tired. It impairs your normal conscious experiences. After about 4 days, both Tripp and Gardner started hallucinating. Tripp thought some spots on a table were insects. Gardner thought a street sign was a person. Soon after that, Gardner, who was white, had a delusional experience in which he thought he was a prominent black athlete (Coren, 1998).

Research in which participants are kept awake for extended periods confirms that sleep deprivation disrupts thinking. When 25 volunteers from the U.S. military were kept awake for 56 hours (Kahn-Greene et al., 2007), they experienced anxiety, depression, and paranoid thinking similar to that experienced by people with severe psychological disorders (see Chapter 16). When a group of medical students was kept awake for 65 hours, they took a long time to answer simple questions and, when answering, spoke with slurred speech (Kamphuisen et al., 1992). Even modest reductions in sleep across a series of days can create ill effects. When the sleeping of a group of adults was restricted to less than 6 hours a night for 14 consecutive days, their ability to think deteriorated; they were less able to pay attention to tasks, responded more slowly to events, and their memory was impaired (Van Dongen et al., 2003).

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There’s a lesson here. If you’ve got an exam coming up, be sure to get plenty of sleep! Inadequate sleep is “the performance killer” (Czeisler & Fryer, 2006). It reduces your alertness and ability to concentrate and solve problems. Even if sleep is merely reduced (e.g., to 5–6 hours) for a few nights in a row, your body seems to “remember” that you got inadequate amounts of sleep on previous nights and you can become as mentally impaired as if you “pulled an all-nighter,” staying up for 24 hours in a row. For example, in a study conducted with a population of healthy young-adult participants, participants whose sleep was restricted to 6 hours a night performed significantly less well on measures of alertness and sustained attention than those who slept for more than 8 hours a night (Lo et al., 2012).

Inadequate sleep impairs performance not only on complex mental activities, such as taking a test, but also on simpler tasks that require sustained attention and quick reactions, such as driving. In the United States, drowsy drivers cause 20% of all automobile accidents (Czeisler & Fryer, 2006). One-fifth of accidents might thus be prevented if everyone got a good night’s sleep!

SLEEP DISORDERS. In the cases of sleep deprivation we just discussed, people who were medically healthy got inadequate amounts of sleep. This lessened sleep resulted from personal choices or social pressures, such as a busy work schedule. A second type of sleep disturbance is different; it results from a medical condition. A sleep disorder is any medical condition that disrupts normal patterns of sleep. A person suffering from a sleep disorder may have difficulty falling asleep or staying asleep, or may suddenly fall asleep at times when they need to stay awake. Let’s look at four main kinds of sleep disorder (American Sleep Apnea Association, 2013; National Heart, Lung, and Blood Institute [NHLBI], n.d.).

Narcolepsy is a relatively rare disorder in which people experience sudden, extreme feelings of sleepiness during the day. Even if they have gotten 8 hours of sleep the night before, people with narcolepsy experience exhaustion, low energy, and mental “cloudiness” during the day. They have trouble staying alert in situations where they are not physically active (such as a classroom lecture). The recurring bouts of sleepiness can significantly interfere with their ability to function normally in school or at work.

Some people with narcolepsy not only feel exhausted, but also fall asleep suddenly and without control. These uncontrollable bouts of sleep tend to be brief “microsleeps” that may last only for a very brief period (Brooks & Kushida, 2002). They nonetheless can be dangerous if people are engaged in an activity such as driving when overcome by sleep.

Narcolepsy is caused by an abnormality in levels of a chemical in the brain that influences wakefulness. Unfortunately, the exact cause of this abnormality is not known and no sure-fire cure is available. Narcolepsy usually is treated by a combined drug and behavioral strategy. Patients may receive a stimulant drug (a drug that increases nervous system activity; see below) while also being instructed to take scheduled naps (Littner et al., 2001).

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Sleep apnea is a disorder in which people suffer from brief pauses in breathing while they are asleep. (The word apnea comes from a Greek word meaning “without breathing.”) Although there are different kinds of sleep apnea, in the most common variety the underlying cause is blockage of airways needed for breathing, especially the passageway at the rear of the throat. When people’s breathing is interrupted, their brain briefly awakens them so that they can breathe normally. These awakenings are necessary to sustain life, yet costly when it comes to a good night’s sleep.

Sleep apnea can have a variety of negative effects. Because their sleep is disturbed, people suffering from sleep apnea may experience the impaired ability to think that characterizes anyone who is sleep deprived, as discussed above. In addition, they are at risk for serious health problems such as heart failure. Sleep apnea disrupts the body’s ability to maintain normal levels of oxygen in the bloodstream. This, in turn, can cause cardiovascular problems that lead to heart failure (Javaheri et al., 1998). Some evidence links the occurrence of sleep apnea to obesity and indicates that weight loss can reduce sleep apnea symptoms (Romero-Corral et al., 2010).

The third sleep disorder is insomnia, which is prolonged difficulty in falling asleep or staying asleep when you have an opportunity to sleep (with that difficulty not caused by one of the other three sleep disorders described above). Everyone has trouble falling asleep on occasion. But some people experience such difficulty night after night, for two weeks in a row or more. These are people suffering from insomnia, a disorder that affects about 10% of the population (Neubauer, 2004).

There are two types of insomnia. In one, disturbed sleep is a side effect of other medical conditions or medications taken to treat them. In the other, insomnia results directly from biological or psychological factors (e.g., stressful daily events). Some foods and drinks, such as caffeine consumed close to bedtime, also can make falling asleep difficult. Alcohol can make falling asleep easier, but causes sleep to be “lighter”; that is, it raises the chance of waking up during the night rather than getting a sound eight hours of sleep. Bedtime environments that are too warm or too noisy can also make sleep harder to achieve (Ohayon & Zulley, 2001).

One treatment for insomnia is sleeping pills, obtained either by prescription or over the counter. They can reduce the time it takes to fall asleep (Dzierzewski et al., 2010; Lemmer, 2007), but have significant drawbacks, including the fact that they can be addictive. Psychological therapies are a second treatment option. In sleep therapies, psychologists educate people about factors that interfere with sleep and teach them techniques that promote a state of relaxation before bedtime (Dzierzewski et al., 2010).

Everybody wants a good night’s sleep, and everybody, at least on occasion, has trouble getting it. Let’s conclude this section on sleep with some sleep tips from the U.S. National Institute on Health (NIH, 2011). If you want a good night’s sleep:

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