Sleep can produce a state of unconsciousness in which the mind and brain apparently turn off the functions that create experience: The theater in your mind is closed. But this is an oversimplification because the theater actually seems to reopen during the night for special shows of bizarre cult films—in other words, dreams. Dream consciousness involves a transformation of experience so radical that it is commonly considered an altered state of consciousness: a form of experience that departs significantly from the normal subjective experience of the world and the mind. Such altered states can be accompanied by changes in thinking, disturbances in the sense of time, feelings of the loss of control, changes in emotional expression, alterations in body image and sense of self, perceptual distortions, and changes in meaning or significance (Ludwig, 1966). Sleep and dreams provide two unique perspectives on consciousness: a view of the mind without consciousness and a view of consciousness in an altered state.

Consider a typical night. As you begin to fall asleep, the busy, task-oriented thoughts of the waking mind are replaced by wandering thoughts and images and odd juxtapositions, some of them almost dreamlike. This presleep consciousness is called the hypnagogic state. On some rare nights, you might experience a hypnic jerk, a sudden quiver or sensation of dropping, as though missing a step on a staircase. No one is quite sure why these jerks happen. Eventually, your presence of mind goes away entirely. Time and experience stop, you are unconscious, and in fact there seems to be no “you” there to have experiences. But then come dreams, whole vistas of a vivid and surrealistic consciousness you just don’t get during the day, a set of experiences that occur with the odd prerequisite that there is nothing “out there” you are actually experiencing. More patches of unconsciousness may occur, with more dreams here and there. And finally, the glimmerings of waking consciousness return again in a foggy and imprecise form as you enter postsleep consciousness (the hypnopompic state) and then awaken, often with bad hair.

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Sleep Cycle

The sequence of events that occurs during a night of sleep is part of one of the major rhythms of human life, the cycle of sleep and waking. This circadian rhythm is a naturally occurring 24-hour cycle, from the Latin circa (about) and dies (day). Even people sequestered in underground buildings without clocks who are allowed to sleep when they want tend to have a rest–activity cycle of about 25.1 hours (Aschoff, 1965).

The sleep cycle is far more than a simple on–off routine, however, as many bodily and psychological processes ebb and flow in this rhythm. EEG recordings reveal a regular pattern of changes in electrical activity in the brain accompanying the circadian cycle. During waking, these changes involve alternation between high-frequency activity (beta waves) during alertness and lower-frequency activity (alpha waves) during relaxation.

The largest changes in EEG occur during sleep. These changes show a regular pattern over the course of the night corresponding to five sleep stages (see FIGURE 5.7). In the first stage of sleep, the EEG moves to frequency patterns even lower than alpha waves (theta waves). In the second stage of sleep, these patterns are interrupted by short bursts of activity called sleep spindles and K complexes, and the sleeper becomes somewhat more difficult to awaken. The deepest stages of sleep are stages 3 and 4, known as slow-wave sleep, in which the EEG patterns show activity called delta waves.

During the fifth sleep stage, REM sleep, a stage of sleep characterized by rapid eye movements and a high level of brain activity, EEG patterns become high-frequency sawtooth waves, similar to beta waves, suggesting that the mind at this time is as active as it is during waking (see FIGURE 5.7). Sleepers wakened during REM periods reported having dreams much more often than those wakened during non-REM periods (Aserinsky & Kleitman, 1953). During REM sleep, the pulse quickens, blood pressure rises, and there are telltale signs of sexual arousal. At the same time, measurements of muscle movements indicate that the sleeper is very still, except for a rapid side-to-side movement of the eyes. (Watch someone sleeping, and you may be able to see the REMs through their closed eyelids. But be careful doing this with strangers down at the bus station.) Although many people believe that they don’t dream much (if at all), some 80% of people awakened during REM sleep report dreams, and most people’s dreams appear to be experienced in real time. Some dreams are also reported in other sleep stages, but not as many—and the dreams that occur at those times are described as less wild than REM dreams and more like normal thinking.

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Putting EEG and REM data together produces a picture of how a typical night’s sleep progresses through cycles of sleep stages (see FIGURE 5.8). In the first hour of the night, you fall all the way from waking to the fourth and deepest stage of sleep, the stage marked by delta waves. These slow waves indicate a general synchronization of neural firing: the neuronal equivalent of “the wave” moving through the crowd at a stadium as lots of individuals move together in synchrony. You then return to lighter sleep stages, eventually reaching REM and dreamland. Note that although REM sleep is lighter than that of lower stages, it is deep enough that you may be difficult to awaken. You then continue to cycle between REM and slow-wave sleep stages every 90 minutes or so throughout the night. Periods of REM last longer as the night goes on, and lighter sleep stages predominate between these periods, with the deeper slow-wave stages 3 and 4 disappearing about halfway through the night. Although you’re either unconscious or dream-conscious at the time, your brain and mind cycle through a remarkable array of different states each time you have a night’s sleep.

Sleep Needs and Deprivation

How much do people sleep? The answer depends on the age of the sleeper (Dement, 1999). Newborns will sleep 6 to 8 times in 24 hours, often totaling more than 16 hours. Their napping cycle gets consolidated into “sleeping through the night,” usually sometime between 9 and 18 months. The typical 6-year-old child might need 11 or 12 hours of sleep, and the average adult needs about 7 to 7.5 hours per night. With aging, people can get along with even a bit less sleep than that. Over a whole lifetime, we get about 1 hour of sleep for every 2 hours we are awake.

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This is a lot of sleeping. Could we tolerate less? For a 1965 science project, 17-year-old Randy Gardner stayed up for 264 hours and 12 minutes. When he finally did go to sleep, he slept only 14 hours and 40 minutes and awakened essentially recovered (Dement, 1978).

Feats like this one suggest that sleep might be expendable. This is the theory behind the classic all-nighter that you may have tried on the way to a rough exam. But it turns out that this theory is mistaken. When people learning a difficult perceptual task are kept up all night after they have finished practicing the task, their learning of the task is wiped out (Stickgold et al., 2000). Sleep following learning appears to be essential for memory consolidation (see Hot Science: Sleep on It, p. 181, in the Memory chapter). It is as though memories normally deteriorate unless sleep occurs to help keep them in place. Studying all night may help you cram for the exam, but it won’t make the material stick, which pretty much defeats the whole point.

Sleep turns out to be a necessity rather than a luxury in other ways as well. At the extreme, sleep loss can be fatal. When rats are forced to break Randy Gardner’s human waking record and stay awake even longer, they have trouble regulating their body temperature and lose weight although they eat much more than normal. Their bodily systems break down and they die, on average, in 21 days (Rechtshaffen et al., 1983). Even for healthy young humans, a few hours of sleep deprivation each night can have a cumulative detrimental effect: reducing mental acuity and reaction time, increasing irritability and depression, and increasing the risk of accidents and injury (Coren, 1997).

Some studies have deprived people of different sleep stages selectively by waking them whenever certain stages are detected. Memory problems and excessive aggression are observed in both humans and rats after only a few days of being wakened whenever REM activity starts (Ellman et al., 1991). Such REM deprivation causes a rebound of more REM sleep the next night (Brunner et al., 1990). Deprivation from slow-wave sleep (in stages 3 and 4), in contrast, has more physical effects, with just a few nights of deprivation leaving people feeling tired, fatigued, and hypersensitive to muscle and bone pain (Lentz et al., 1999).

It’s clearly dangerous to neglect the need for sleep. But why would we have such a need in the first place? All animals appear to sleep, although the amount of sleep required varies quite a bit from species to species (see FIGURE 5.9). Giraffes sleep less than 2 hours daily, whereas brown bats snooze for almost 20 hours. These variations in sleep needs—and the very existence of a need—are hard to explain. Is the restoration that happens during the unconsciousness of sleep something that simply can’t be achieved during consciousness? Sleep is, after all, potentially costly in the course of evolution. The sleeping animal is easy prey, so the habit of sleep would not seem to have developed so widely across species unless it had significant benefits that made up for this vulnerability. Theories of sleep have not yet determined why the brain and body have evolved to need these recurring episodes of unconsciousness.

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Sleep Disorders

In answer to the question, “Did you sleep well?,” comedian Stephen Wright said, “No, I made a couple of mistakes.” Sleeping well is something everyone would love to do, but for many people, sleep disorders are deeply troubling.

Insomnia, difficulty in falling asleep or staying asleep, is perhaps the most common sleep disorder. About 30-48% of people report symptoms of insomnia, and 6% of people meet criteria for a diagnosis of insomnia, a diagnosis that requires persistent and impairing sleep problems (Bootzin & Epstein, 2011; Ohayon, 2002). There are many potential causes of insomnia. In some instances, it results from lifestyle choices such as working night shifts (self-induced insomnia); sometimes it occurs in response to depression, anxiety, or some other condition (secondary insomnia); and in other cases, there are no obvious causal factors (primary insomnia). Regardless of type, insomnia can be exacerbated by worrying about insomnia (Borkevec, 1982). No doubt you’ve experienced some nights when sleeping was a high priority, such as before a class presentation or an important interview, and you’ve found that you were unable to fall asleep. The desire to sleep initiates an ironic process of mental control—a heightened sensitivity to signs of sleeplessness—and this sensitivity interferes with sleep. Although sedatives can be useful for brief sleep problems associated with emotional events, their long-term use is not effective. Not only are most sleeping pills addictive, but even in short-term use, sedatives can reduce the proportion of time spent in REM and slow-wave sleep (Qureshi & Lee-Chiong, 2004). As a result, the quality of sleep achieved with pills may not be as high as without, and there may be side effects such as grogginess and irritability during the day.

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Sleep apnea is a disorder in which the person stops breathing for brief periods while asleep. A person with apnea usually snores because apnea involves an involuntary obstruction of the breathing passage. When episodes of apnea occur for over 10 seconds at a time and recur many times during the night, they may cause many awakenings and sleep loss or insomnia. Apnea occurs most often in middle-age overweight men (Punjabi, 2008) and may go undiagnosed because it is not easy for the sleeper to notice. Bed partners may be the ones who finally get tired of the snoring and noisy gasping for air when the sleeper’s breathing restarts, or the sleeper may eventually seek treatment because of excessive sleepiness during the day. Therapies involving weight loss, drugs, sleep masks that push air into the nasal passage, or surgery may solve the problem.

Somnambulism (or sleepwalking) occurs when a person arises and walks around while asleep. Sleepwalking is more common in children, peaking between the ages of 4 and 8 years, with 15-40% of children experiencing at least one episode (Bhargava, 2011). Sleepwalking tends to happen early in the night, usually in slow-wave sleep, and sleepwalkers may awaken during their walks or return to bed without waking, in which case they will probably not remember the episode in the morning. The sleepwalker’s eyes are usually open in a glassy stare. Walking with hands outstretched is uncommon except in cartoons. Sleepwalking is usually only problematic in that sleepwalkers sometimes engage in strange or unwise behaviors such as urinating in places other than the toilet and leaving the house while still sleeping. People who walk while they are sleeping do not tend to be very coordinated and can trip over furniture or fall down stairs. After all, they’re sleeping. Contrary to popular belief, it is safe to wake sleepwalkers or lead them back to bed (but best to wait until after they finish their business).

There are other sleep disorders that are less common. Narcolepsy is a disorder in which sudden sleep attacks occur in the middle of waking activities. Narcolepsy involves the intrusion of a dreaming state of sleep into waking and is often accompanied by unrelenting excessive sleepiness and uncontrollable sleep attacks lasting from 30 seconds to 30 minutes. This disorder appears to have a genetic basis, as it runs in families, and it can be treated effectively with medication. Sleep paralysis is the experience of waking up unable to move and is sometimes associated with narcolepsy. This eerie experience usually happens as you are awakening from REM sleep but before you have regained motor control. This period typically lasts only a few seconds or minutes and can be accompanied by hypnopompic (when awakening) or hypnagogic (when falling asleep) hallucinations in which dream content may appear to occur in the waking world. A very clever series of recent studies suggests that sleep paralysis accompanied by hypnopompic hallucinations of figures in one’s bedroom seems to explain many perceived instances of alien abductions and recovered memories of sexual abuse (aided by therapists who used hypnosis to help the sleepers [incorrectly] piece it all together; McNally & Clancy, 2005). Night terrors (or sleep terrors) are abrupt awakenings with panic and intense emotional arousal. These terrors, which occur most often in children and in only about 2% of adults (Ohayon, Guilleminault, & Priest, 1999), happen most often in non-REM sleep early in the sleep cycle and do not usually have dream content the sleeper can report.

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To sum up, there is a lot going on when we close our eyes for the night. Humans follow a pretty regular sleep cycle, going through the five stages of sleep during the night. Disruptions to that cycle, either from sleep deprivation or sleep disorders, can produce consequences for waking consciousness. But something else happens during a night’s sleep that affects our consciousness, both while asleep and when we wake up.

Pioneering sleep researcher William C. Dement (1959) said, “Dreaming permits each and every one of us to be quietly and safely insane every night of our lives.” Indeed, dreams do seem to have a touch of insanity about them. Even more bizarre is the fact that we are the writers, producers, and directors of the crazy things we experience in dreams. Just what are these experiences, and how can they be explained?

Dream Consciousness

Dreams depart dramatically from reality. You may dream of being naked in public, of falling from a great height, of sleeping through an important appointment, or of being chased (Holloway, 2001). These things don’t happen much in reality unless you’re having a very bad life. The quality of consciousness in dreaming is also altered significantly from waking consciousness. There are five major characteristics of dream consciousness that distinguish it from the waking state (Hobson, 1988).

Not all of our dreams are fantastic and surreal, however. We often dream about mundane topics that reflect prior waking experiences or “day residue.” Current conscious concerns pop up (Nikles et al., 1998), along with images from the recent past. The day residue does not usually include episodic memories—that is, complete daytime events replayed in the mind. Rather, dreams that reflect the day’s experience tend to single out sensory experiences or objects from waking life and mix them together. For instance, after a fun day at the beach with your roommates, your dream that night might include cameo appearances by bouncing beach balls or a flock of seagulls. One study had research participants play the computer game Tetris and found that participants often reported dreaming about the Tetris geometrical figures falling down—even though they seldom reported dreams about being in the experiment or playing the game (Stickgold et al., 2001). The content of dreams takes snapshots from the day rather than retelling the stories of what you have done or seen. This means that dreams often come without clear plots or story lines, so they may not make a lot of sense.

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Some of the most memorable dreams are nightmares. One set of daily dream logs from college undergraduates suggested that the average student has about 24 nightmares per year (Wood & Bootzin, 1990), although some people may have them as often as every night. Children have more nightmares than adults, and people who have experienced traumatic events are inclined to have nightmares that relive those events. Following the 1989 earthquake in the San Francisco Bay area, for example, college students who had experienced the quake reported more nightmares than those who had not, and they often reported that the dreams were about the quake (Wood et al., 1992).

Dream Theories

Dreams are puzzles that cry out to be solved. The search for dream meaning goes all the way back to biblical figures, who interpreted dreams and looked for prophecies in them. In the Old Testament, the prophet Daniel (a favorite of three of the authors of this book) curried favor with King Nebuchadnezzar of Babylon by interpreting the king’s dream. Unfortunately, the meaning of dreams is usually far from obvious.

In the first psychological theory of dreams, Freud (1900/1965) proposed that dreams are confusing and obscure because the dynamic unconscious creates them precisely to be confusing and obscure. According to Freud’s theory, dreams represent wishes, and some of these wishes are so unacceptable, taboo, and anxiety producing that the mind can only express them in disguised form. For example, a dream about a tree burning down in the park across the street from where a friend once lived might represent a camouflaged wish for the death of the friend. In this case, wishing for the death of a friend is unacceptable, so it is disguised as a tree on fire. The problem with Freud’s approach is that there is an infinite number of potential interpretations of any dream, and finding the correct one is a matter of guesswork—and of convincing the dreamer that one interpretation is superior to the others.

Although dreams may not represent elaborately hidden wishes, there is evidence that they do feature the return of suppressed thoughts. Researchers asked volunteers to think of a personal acquaintance and then to spend 5 minutes before going to bed writing down whatever came to mind (Wegner, Wenzlaff, & Kozak, 2004). Some participants were asked to suppress thoughts of this person as they wrote, others were asked to focus on thoughts of the person, and yet others were asked just to write freely about anything. The next morning, participants wrote dream reports. Overall, all participants mentioned dreaming more about the person they had named than about other people. But they most often dreamed of the person they named if they were in the group that had been assigned to suppress thoughts of the person the night before. This finding suggests that Freud was right to suspect that dreams harbor unwanted thoughts. Perhaps this is why actors dream of forgetting their lines, travelers dream of getting lost, and football players dream of fumbling the ball.

Another key theory of dreaming is the activation-synthesis model (Hobson & McCarley, 1977). This theory proposes that dreams are produced when the brain attempts to make sense of random neural activity that occurs during sleep. During waking consciousness, the mind is devoted to interpreting lots of information that arrives through the senses. You figure out that the odd noise you’re hearing during class is your cell phone vibrating, for example, or you realize that the strange smell in the hall outside your room must be from burned popcorn. In the dream state, the mind doesn’t have access to external sensations, but it keeps on doing what it usually does: interpreting information. Because that information now comes from neural activations that occur without the continuity provided by the perception of reality, the brain’s interpretive mechanisms can run free. This might be why, for example, a person in a dream can sometimes change into someone else. There is no actual person being perceived to help the mind keep a stable view. In the mind’s effort to perceive and give meaning to brain activation, the person you view in a dream about a grocery store might seem to be a clerk but then change to be your favorite teacher when the dream scene moves to your school. The great interest people have in interpreting their dreams the next morning may be an extension of the interpretive activity they’ve been doing all night.

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The Freudian theory and the activation–synthesis model differ in the significance they place on the meaning of dreams. In Freud’s theory, dreams begin with meaning, whereas in the activation–synthesis model, dreams begin randomly—but meaning can be added as the mind lends interpretations in the process of dreaming. Dream research has not yet sorted out whether one of these theories or yet another might be the best account of the meaning of dreams.

The Dreaming Brain

What happens in the brain when we dream? Several fMRI studies show that the brain changes that occur during REM sleep correspond clearly with certain alterations of consciousness that occur in dreaming. FIGURE 5.10 shows some of the patterns of activation and deactivation found in the dreaming brain (Nir & Tononi, 2010; Schwartz & Maquet, 2002). Many dreams have emotional content—minor worries, the occasional monster, and sometimes that major exam you’ve forgotten about until you walk into class. And as it turns out, the amygdala, which is involved in response to threatening or stressful events, is quite active during REM sleep. However, even though the typical dream is a visual wonderland, the areas of the brain responsible for visual perception are not activated during dreaming. Instead, the visual association areas in the occipital lobe that are responsible for visual imagery do show activation (Braun et al., 1998). Your brain is smart enough to realize that it’s not really seeing bizarre images but acts instead as though it’s imagining bizarre images. Meanwhile, the prefrontal cortex—usually associated with planning and executing actions—tends to show less activity during REM sleep than it does during waking consciousness, which may help explain why dreams often seem to be unplanned and rambling.

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Another odd fact of dreaming is that while the eyes are moving rapidly, the body is otherwise very still. During REM sleep, the motor cortex is activated, but spinal neurons running through the brain stem inhibit the expression of this motor activation (Lai & Siegal, 1999). This turns out to be a useful property of brain activation in dreaming; otherwise, you might get up and act out every dream! People who are moving during sleep are probably not dreaming. The brain specifically inhibits movement during dreams, perhaps to keep us from hurting ourselves.