What is the significance of the eye movements that occur while we sleep?

Much of what we know about sleep is derived from research conducted in sleep laboratories throughout the world since World War II. Researchers representing several disciplines, including neurology, psychiatry, and psychology, have probed thousands of individuals in search of solutions to the mysteries of sleep and dreams.

Hans Berger’s invention of the electroencephalograph (EEG) made it possible to study the living brain without entering it. Although the EEG was a significant advance, the major breakthrough in studying sleep was the observation of the eyes of sleeping people. Nathaniel Kleitman gave the task of directly observing eye movements in sleeping individuals to a graduate student, Eugene Aserinsky. Electrodes attached next to the eyes of the sleeping individuals provided a continuous record of periods during which the eyes moved back and forth (Aserinsky & Kleitman, 1953). Researchers coined the term rapid eye movement (REM) to describe this phenomenon.

A Night in a Sleep Lab

A posted request for sleep study volunteers catches your attention. Imagine being paid for sleeping! You telephone the researcher, who tells you that she is interested in the sleep patterns of college students with part-time jobs. You arrive two hours before your usual bedtime. A technician begins to attach several dime-sized electrodes to different parts of your body. The electrodes detect physiological changes that occur during the night. The information is fed to a polysomnograph, an instrument that produces a continuous paper record of these physiological changes.

The technician attaches several electrodes to your scalp to measure brain waves. Two more electrodes, one near each eye, provide data for the electrooculograph (EOG), which detects eye movements. An electrode under your chin measures muscle activity; your heart rate is monitored by an electrocardiograph (EKG). When other devices that measure breathing, oxygen in the blood, and temperature are added, you look and feel like a robot.

You are ushered into a soundproof room, where every movement and sound you make will be recorded. Apprehensive about possible equipment malfunction, you find fearful thoughts running through your mind. The first night’s sleep is so unlike a typical night at home that researchers usually discard the data. The next night, having adapted to your new surroundings, you sleep comfortably.

The Stages of Sleep

During a typical night, we pass through five stages of sleep: REM sleep and four other stages of sleep known as Stages 1, 2, 3, and 4. During each stage of sleep, a distinctive "signature" of brain waves appears on the EEG record (see Figure 6-3). Let’s take a closer look at these stages.

NREM Sleep. 
We begin sleep with Stage 1 and then progress to Stages 2, 3, and 4. At each step, we become more difficult to awaken. These four stages are known collectively as non-REM (NREM) sleep. You typically spend a few minutes in Stage 1, a transitional phase between wakefulness and sleep that is described as drowsiness. Stage 2 sleep occupies about 50 percent of an adult’s sleep. The EEG heralds the arrival of Stage 2 with the appearance of sleep spindles—bursts of activity on the EEG record (see Figure 6-3. Finally, we descend to Stage 3 and 4, which together are known as slow-wave sleep or delta sleep; they are believed to be the most restorative sleep stages. Waking someone from slow-wave sleep is quite difficult; individuals awakened from this deep sleep are likely to be disoriented and groggy. Delta waves first appear in Stage 3, in which they account for 20 to 50 percent of the brain waves. By Stage 4 sleep, delta waves make up more than 50 percent of the brain waves. Sleep patterns may vary from one person to another, but we can generally count on the progression of sleep stages outlined in Figure 6-4. As you can see, from the deep sleep of Stages 3 and 4 we ascend through the lighter NREM stages and then enter a dramatically different stage of sleep called REM.

REM Sleep. 
What makes REM sleep so different from the other sleep stages? We know that rapid eye movements are associated with dreaming, but that is just part of the story. At one time, sleep was viewed as a "timeout," like turning off your car’s motor; this view is wrong (Dement, 1986). During rapid eye movement (REM) sleep, the brain is more like a parked car with its motor racing. The signs of activity are all over: Heart rate and respiration are more variable than they are during NREM stages. Men have penile erections; women experience an engorgement of blood in the genital area. However, this indication of sexual arousal is not related to erotic dreams. The EEG record of a person in REM sleep resembles that of an awake person (review Figure 6-3). Intense activity of neurons throughout the brain make a PET scan  look like a lit Christmas tree; brain temperature increases as more blood flows to the brain . As we saw in Figure 6-4, adults spend 20 to 25 percent of the night in this supercharged stage of sleep.

The lab technician attached an electrode to your chin to detect muscle activity. Why? During REM sleep, activity in your skeletal (voluntary) muscles is suppressed, leaving you essentially paralyzed; this paralysis shows up first in your chin and neck. This curious combination of an active brain with inactive muscles led researchers to describe REM as "paradoxical sleep."

The end of the first REM period marks the end of a sleep cycle—the period from the beginning of sleep to the end of REM sleep-which takes about 90 minutes. The sleep cycle repeats itself, with some changes, four to six times a night in most adults (review Figure 6-4).

Changes in the sleep cycle during the night involve slow-wave and REM sleep. The amount of slow-wave sleep decreases during the night; most of your slow -wave sleep occurs early. The length of REM episodes increases during the night: The first REM period may be five to ten minutes long; the final one can last 30 minutes or more.

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Differences in Individual Sleep Patterns

As you can see in Figure 6-6, the amount of time people sleep varies with age. During the first week of life, most infants sleep about 16 hours a day. However, some newborns sleep as little as 11 hours and others more than 20. These large differences are likely due to genetic factors; sleep length is more similar in identical twins than in fraternal twins (Heath et al., 1990; Webb & Campbell, 1983).

Sleep length decreases with age through adolescence, but it does not change much from early adulthood to the seventies. Should you worry if you don’t sleep seven or eight hours, as the average adult does? When it comes to sleep, one size does not fit all. Two of every ten adults sleep less than six hours a night, and one in ten sleeps nine hours or more (Hauri & Linde, 1990). How do you assess your own need for sleep? The key question to ask is, Are you refreshed after you awaken? For an increasing number of individuals, the answer is no!

The multiple sleep latency test is a sensitive indicator of an individual’s level of sleepiness. When using this technique, researchers ask you to try to fall asleep every two hours during normal waking hours. If you fall asleep, they wake you and repeat the request later. Falling asleep easily is a sign that your daily amount of sleep does not satisfy your need. Numerous such observations have convinced some sleep experts that "most people no longer know what it feels like to be fully alert" (Coleman, 1995, p. 67). Current levels of sleepiness are epidemic; most of us need more sleep than we are getting (Dement, 1992). Most adults now sleep one hour less than they did at the beginning of the century (Webb, 1992). The sleep time reported by college students has dropped from 7.3 hours in 1978-1979 to 6.9 hours in 1988-1989 (Hicks et al., 1990).

Contrary to common belief, older people do not need less sleep than younger adults. However, they do awaken more often during the night; consequently, their sleep is more fragmented (see Figure 6-7). The sleep efficiency index--the proportion of bed time actually spent sleeping—reveals what happens to sleep as we grow older. Sleep efficiency is above 95 percent for men and women through their thirties; however, it drops to about 80 percent among individuals in their seventies. Sleep efficiency drops in older people for two reasons. First, the deep sleep of Stage 4 is either markedly reduced or completely absent. Second, elderly people spend more time in the lighter stages of sleep, which makes them more susceptible to awakening.

Most adults have just one period of sleep every 24 hours; babies have six to eight sleep periods a day. The next time you hear someone say that he or she "slept like a baby," point out that a baby’s sleep consists of several short episodes of sleep. Over time those short episodes consolidate (see Figure 6-8); by 6 months of age, about 80 percent of babies sleep through the night (Webb, 1992).

Toddlers are likely to have two episodes of sleep a day (night sleep and an afternoon nap). Most children stop napping when they attend elementary school, yet as many as 50 percent of college students take naps either every day or occasionally to compensate for lost nighttime sleep. In addition to college students, many other normal healthy adults nap. Some famous nappers include Wolfgang Mozart, Napoleon Bonaparte, and Winston Churchill. Many adults nap once or more a week, most often when their night sleep is inadequate. Although some nappers experience sleep inertia, a temporary feeling of impairment that follows awakening, naps typically improve both mood and performance (Dinges, 1989).

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Sleep Deprivation

If you live to be 75 years old, you will spend one third of your life (about 220,000 hours) sleeping. Why? One way to answer this question is to deprive people of sleep and watch what happens.

In January 1959, a 32-year-old disc jockey, Peter Tripp, decided to go without sleep for 200 hours as a publicity stunt to raise money for charity. Undaunted by experts who warned of the risk of death, Tripp persevered. When he found it extremely difficult to stay awake after 135 hours, he turned to stimulants. The stimulants (amphetamines) altered his perceptions: Specks of dust became insects; a bureau drawer burst into flames. Yet he hosted his show, "Your Hits of the Week," between 5 and 8 p.m. each day, without giving any hint of what he was enduring. After 201 hours without sleep, he slept for 13 hours; the only symptom he experienced as a result of his ordeal was a slight depression that lasted a few months.

In 1964, Randy Gardner, a 17-year-old California high school student, resolved to set a new world record for sleep deprivation. Gardner used no stimulants--not even coffee—during his 264 hours without sleep (Gulevich, Dement, & Johnson, 1966). On the first night after setting the record, he slept less than 15 hours.*

These individuals extended the limits of sleep deprivation without suffering any apparent serious long-term consequences. In fact, "systematic studies of total sleep deprivation in humans revealed no permanent effects and few profound deficits"
(Anch et al., 1988, p. 9).

Both Peter Tripp and Randy Gardner appeared to go without sleep for several days. We now know that total sleep deprivation is almost impossible except in rare neurological disorders. Individuals who undergo long periods of sleep deprivation experience lapses of attention, forgetfulness, and impaired performance. Why? The answer seems to be microsleeps, which are episodes of sleep (up to 30 seconds) that intrude on wakefulness. As the period of sleep deprivation gets longer, the microsleeps become longer and more frequent; total sleep deprivation does not occur for very long in humans. Most cases of total sleep deprivation are not really total. Although microsleeps do not compensate for all of the lost sleep, they reveal how powerful the need for sleep can be when we try to deny it for extended periods.

*Robert McDonald of California holds the world record for sleeplessness; in 1986 he rocked in a rocking chair for 453 hours, 40 minutes (McFarlan, 1991).

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The Function of Sleep

What purpose does sleep serve? The answer to this seemingly easy question has proved elusive. Attempts to answer the question have focused on REM sleep and slow-wave sleep as well as on the overriding question of why we sleep at all.

Researchers have deprived people of REM sleep by waking them whenever they  entered this stage. When REM-deprived people were allowed to sleep without  interruption on subsequent nights, they exhibited an increased percentage of REM sleep, a phenomenon known as the REM rebound. The body apparently needs REM sleep because it attempts to recapture some if it is lost. But demonstrating a need does not tell us what purpose it serves.

There is a clue in the fact that half of a newborn’s 16 hours of sleep per day is spent in REM; premature babies spend even more than half of their sleep in REM (Roffwarg, Muzio, & Dement, 1966). Why do babies spend so much time in REM sleep? REM sleep—a period of high activity in the brain—may provide babies with the stimulation they cannot provide for themselves. This stimulation may be necessary for the development of the brain. The function of REM sleep in adults is still not certain; however, accumulating evidence suggests that it plays a role in consolidating or strengthening memories (Barinaga, 1994).

Myth Or Science

Evidence suggesting that memories may be consolidated during REM sleep leads us to ask this question: Can we learn new material while asleep? The popular press has heralded sleep learning as an easy way to learn. Imagine playing a tape of course material while you sleep and waking the next morning with the material in your memory. Unfortunately, laboratory evidence reveals that although we may process some sensory information during sleep, as when we awaken to the sound of a fire alarm or a crying baby, we do not retain information for later recall. Some individuals who appeared to retain information from their sleep were actually awake, based on EEG records, during the playing of the tape. Thus sleep learning appears to be a myth born of wishful thinking (Badia, 1993).

The commonsense notion that sleep restores fatigued bodies is difficult to reconcile with certain facts. If you spend most of the day in bed, for example, you will still sleep that night even though your energy expenditure and wear and tear on your body have been minimal. This observation reflects the built-in nature of the circadian sleep-wake cycle, which causes sleepiness at about the same time every day. However, starvation, surgery, and other physical demands sometimes lead to increases in slow-wave sleep. This deep sleep is the stage during which rest and repair are likely to occur. In addition, our body secretes the greatest amount of growth hormone during slow-wave sleep.

But why do we sleep at all? Wilse Webb (1988) suggests that sleep developed in each species in ways that increased survival. Whether a species adapts best by sleeping a lot or a little depends on two factors: the animal’s vulnerability to predators and its need for food. Animals that are at great risk from predators must be alert because periods of inactivity put them in grave danger. Deer and sheep do not sleep much. By contrast, lions may sleep in open areas for two or three days after consuming a meal of gazelle or antelope because they have so few enemies (Hobson, 1989).

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Sleep Problems

The growing number of clinics that diagnose and treat sleep disorders attests to the fact that sleep is not always peaceful and restful. Sleep disorders range from annoying to life threatening and can be divided into three categories: insomnia, hypersomnias, and parasomnias.

Insomnia.

"I couldn’t sleep at all last night; I was tossing and turning." If this condition sounds familiar, you are not alone. Approximately 35 percent of adults experience this condition, termed insomnia, each year, and about half of them view it as serious (Mellinger, Balter, & Uhlenhuth, 1985). Insomnia is a very common complaint, but fortunately it usually lasts less than three weeks (Reite, Nagel, & Ruddy, 1990).

Insomnia can take many forms: difficulty falling asleep, waking up too early, multiple awakenings, poor-quality sleep. Most cases of insomnia are related to life stress or the disruption of circadian rhythms due to jet lag or nighttime work. Too much caffeine and illnesses that result in physical or psychological discomfort are also significant causes of brief bouts of insomnia (Dryer & Kaplan, 1986; Pagel, 1994). Chronic cases are often associated with emotional problems, especially anxiety and depression (Buysse et al., 1994; Pagel, 1994).

The most common form of insomnia is difficulty falling asleep, known as sleep onset insomnia (Pillitteri et al., 1994). Most people fall asleep within 15 minutes after their head hits the pillow; others toss and turn. This type of insomnia is usually associated with stressful events and disappears when those events end. We may toss and turn the night before an important interview, but the next night we have no trouble sleeping. However, if the insomnia continues and becomes chronic, we search for a cure. We take pills for all kinds of other ailments, so why not for sleep problems?

Over-the-counter (nonprescription) sleep aids are frequently used for insomnia (Pillitteri et al., 1994). These drugs contain an antihistamine that causes drowsiness that can extend into waking hours and can thus affect our ability to drive.

A number of prescription drugs are available to treat insomnia. Although sleeping pills do not cure insomnia, they do provide relief from the symptoms (Gillin & Byerley, 1990). However, the use of sedative or hypnotic drugs to treat insomnia has declined as a result of growing recognition of their side effects and other problems (Pagel, 1994). For example, if you use them every night, they may lose their effectiveness in a few weeks as your body adjusts to them. Sleeping pills that reduce REM sleep can cause a REM rebound when you stop taking them. During succeeding nights, insomnia may return along with nightmares produced by the additional REM sleep.

Several psychological treatments such as relaxation and stimulus control are quite effective in reducing sleep latency (time it takes to fall asleep) and increasing sleep time (Murtagh & Greenwood, 1995). The stimulus control method--used for sleep onset insomnia—is a set of rules designed to establish better sleeping habits. For example, people with sleep onset insomnia are instructed to lie down to sleep only when sleepy, not to use the bed for anything except sleep and sex, and to get out of bed if unable to sleep (Bootzin, Epstein, & Wood, 1991). These suggestions are also elements of good sleep hygiene; in fact, the best way to deal with insomnia is to practice good sleep hygiene. For example, maintain a regular sleep schedule, and avoid caffeine. Table 5-1 lists several rules to help you sleep better.

Hypersomnias.

Although our culture does not recognize excessive daytime sleepiness as a serious complaint, it can be a symptom of a serious medical disorder. Hypersomnias are sleep disorders that are characterized by excessive daytime sleepiness. They include narcolepsy and sleep apnea.

Imagine that every day you feel sleepy. Your eyes droop as you struggle to remain awake, but sleep wins, and your head plunges to your chest. Excessive daytime sleepiness, regardless of the amount of nighttime sleep, is usually the first symptom of narcolepsy (narce, "numbing"; lepsis, "attack"), a lifelong sleep disorder that afflicts about 300,000 Americans (American Narcolepsy Association, n.d.). A person with narcolepsy may fall asleep in a few minutes during a multiple sleep latency test.

Now imagine a friend who suddenly collapses to the floor while telling you a joke. Your friend has just experienced one of the symptoms of narcolpesy: cataplexy (cata, "down"; plexis, "strike")--a sudden loss of muscle tone often triggered by strong emotion. Attacks of cataplexy range from partial muscle weakness to an almost complete loss of muscle tone that lasts between a few seconds and a minute. Two other symptoms of narcolepsy are hypnagogic hallucinations—intense, vivid dreams that occur at the beginning of sleep—and paralysis at the beginning of sleep or upon awakening.

One victim of narcolepsy continually fell asleep while in high school and college. Repeated bouts of sleep led to 15 automobile accidents by age 25; fortunately, no one was seriously injured (Fritz, 1993). Narcolepsy usually begins in adolescence or early adulthood; however, 10 to 15 years may elapse before a correct diagnosis is made.

Sleep Apnea

Another major hypersomnia is seen in the case of a 59-year-old lawyer who fell asleep during a conference with a client. His spouse provided the clue to the diagnosis: Her husband’s snoring was so loud that she moved to another room (Hartmann, 1987). Between 10 and 30 percent of adults snore with no serious consequences. However, for about 2 percent of middle-aged women and 4 percent of middle-aged men, loud snoring can be a sign of a serious sleep disorder known as sleep apnea (from the Greek word for "absence of breathing"; Young et al., 1993). In sleep apnea, the flow of air to the lungs stops for at least 10 to 15 seconds due to lack of effort by the diaphragm or collapse of the airway (Brock & Shucard, 1994). After going without oxygen for as long as 90 seconds, the brain signals an emergency wake-up to fill the lungs with air. Apnea victims may awaken as many as 1,000 times a night, yet they become accustomed to the multiple awakenings. The risk of developing sleep apnea is higher among males and obese individuals (Brock & Shucard, 1994; Young et al., 1993); sleep apnea also substantially increases their risk of death (Schmidt-Nowara & Jessop, 1995).

A very effective treatment for apnea is continuous positive airway pressure (CPAP, pronounced "see-pap"). At night, a mask placed over the patient’s nose is connected by a tube to a machine that maintains a flow of pressured air to the lungs to prevent airway collapse. Although CPAP therapy is effective, many patients stop using it because it is uncomfortable (National Commission on Sleep Disorders Research, 1992). A more radical treatment, tracheotomy—which involves making an incision in the trachea—allows a victim to breathe during the night by bypassing the collapsed airway. In other cases, some soft tissue at the back of the throat is surgically removed to enlarge the breathing passage.

Parasomnias.

The parasomnias are undesirable behaviors that occur exclusively during sleep or are exacerbated by sleep. This diverse group of sleep disorders ranges from bedwetting to nightmares. They occur more frequently in children than in adults, perhaps reflecting the immaturity of the child’s nervous system. Although many parasomnias disappear in time without treatment, some are potentially dangerous and even deadly.

SIDS.

Each year, more than 5,000 apparently healthy infants are found dead in their cribs after a night’s sleep or a midday nap. Sudden infant death syndrome (SIDS) is the sudden death of an infant under one year of age that is not explained by an autopsy and investigation of the child’s history and death scene. SIDS is the leading cause of death between the ages of one week and one year.

We do not know what causes SIDS, but do know that certain factors increase the likelihood that it will occur. Sleeping in the prone position increases the risk of SIDS, as do a soft mattress, a recent illness, and elevated room temperature. The specific means by which these factors contribute to SIDS is not yet known. One possibility is that the soft bedding material obstructs the air passage. Another possibility is that soft mattresses reduce air movement, which can increase the likelihood that the baby will rebreathe the carbon dioxide that was just exhaled (Ponsonby et al., 1993). Infants who have died of SIDS have a deficiency in the binding of acetycholine in the medulla—the part of the brain that controls breathing. This deficiency seems to render these infants less able to respond with protective reflexes that ensure proper breathing when their oxygen level drops (Kinney et al., 1995).

Public health campaigns in several countries have encouraged caregivers to place infants to sleep on their side or back. These campaigns have led to decreases of approximately 50 percent in the SIDS death rate (Dwyer et al., 1995; Willinger, 1995). A national public education campaign, using the slogan "Back to sleep," is supported by a coalition of organizations including the U.S. Public Health Service (Willinger, 1995). In addition, some caregivers now use electronic monitors that track an infant’s breathing and sound an alarm if it should stop.

Sleepwalking.

As many as 20 percent of the population, mostly children between the ages of 4 and 12, have experienced one or more episodes of sleepwalking, or somnambulism (Anch et al., 1988). In the movies, sleepwalkers walk about with their arms stretched out in front of them. In real life, the typical episode involves merely sitting up in bed. When sleepwalkers leave their beds, they are likely to stumble about in a disoriented state and may turn on the lights, eat cat food sandwiches, or walk through open doors or windows. Although their eyes are likely to be open, the sleepwalker’s EEG shows an unusual mixture of the brain waves associated with deep sleep and relaxed wakefulness.

Although sleepwalking is closely associated with slow-wave sleep, another sleep disorder that occurs out of REM sleep resembles sleepwalking. REM sleep behavior disorder (RBD) is a syndrome of injurious or disruptive behavior that emerges during REM sleep (Schenck & Mahowald, 1990). RBD occurs primarily in older men; the average age of onset is in the fifties (Schenck, Hurwitz, & Mahowald, 1993). The behavior may consist of sitting up, jumping out of bed, running, and punching. As a result, both victim and spouse are frequently injured. One suspected cause is subtle age-related changes in the brain that affect the mechanism that usually suppresses voluntary muscle activity during REM sleep.

Here is some helpful advice for parents of sleepwalkers: Remove any electrical cords that might cause tripping; locate the sleepwalker’s bedroom on the first floor; lock windows and doors. In most cases sleepwalking occurs for a few years and then disappears without treatment. If the sleepwalking persists into adulthood, however, there is reason for concern; such cases have been associated with several psychiatric disorders (Reite, Nagel, & Ruddy, 1990). If the diagnosis is RBD, it can be treated effectively with a mild tranquilizer.

Enuresis.

Enuresis (bedwetting) is a childhood sleep problem that affects about 5 million children in the United States. Sleep specialists do not consider it a disorder unless the child is at least 5 years old (Friman & Warzak, 1990). Like other childhood sleep disorders, enuresis may reflect an immature nervous system. The tendency to develop enuresis runs in families; children with enuresis may inherit a delay in the development of the bladder muscles needed to control urination while sleeping.

Drug treatment of enuresis can pose health risks, and the drugs are often ineffective in the long run (Friman & Warzak, 1990). Other approaches to dealing with enuresis include waking the child periodically throughout the night and rewarding the child for sleeping through the night without wetting the bed. The most common and apparently the best treatment is the "pad and buzzer," or urine alarm (Friman & Warzak, 1990; Houts, 1991). A urine sensor placed under the bedsheets or sewn into the child’s underpants is connected to a buzzer that sounds when the first drops of urine fall on it. The resultant awakenings make the child aware of bladder pressure. Once children have gained this awareness, they awaken spontaneously before beginning to urinate.

Sleep Terrors/Night Terrors.

Sleep terrors (also called night terrors) are intensely frightening experiences that occur during Stage 4 sleep. About 5 percent of children between ages 2 and 5 experience sleep terrors; the disorder usually disappears as the child matures. The first sign of a sleep terror is often a blood-curdling scream, usually followed by sitting up in bed. The accompanying physiological arousal is remarkable: Heart rate can triple in a minute, breathing is labored, and the sleeper is soaked with perspiration. Children are unable to recall the experience, which often terrifies their parents. In contrast, adult victims of night terrors (who have the experience very rarely) tend to recall the event in detail.

Nightmares are frightening dreams that occur during REM sleep; however, they are mild compared to sleep terrors. The dreamer experiences moderate anxiety and can often recall the disturbing dream. Young adults seem to experience about one nightmare per month. Although nightmare frequency in the general population is not related to anxiety levels (Wood & Bootzin, 1990), lifelong frequent sufferers of nightmares do exhibit symptoms of psychological disturbance (Berquier & Ashton, 1992).

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Dreams: Nighttime Theater

Psychologist Calvin Hall (1966) describes a dream as "a succession of images, predominantly visual in quality, which are experienced during sleep" (p. 3). Opinions vary about the importance we should attach to this succession of images, but Hall believes that interpreting our dreams can enhance our self -knowledge.

As typically defined, dreams occur during REM sleep; however, do not let the association between REM and dream reports lead you to conclude that NREM is a mental wasteland. If you wake people from NREM sleep, they are likely to report some mental content. However, there are qualitative differences between REM and NREM reports. REM dream reports are more vivid, more visual, more dramatic, more emotional, and more active. NREM reports tend to be just the opposite; they often resemble thoughts and are more concerned with current problems than REM dreams.

Why We Forget Our Dreams.

Dream recall varies from one person to another. To study why we forget our dreams, psychologist David Cohen randomly divided college students into two groups and asked both groups to write down their dreams upon awakening (Cohen & Wolfe, 1973). Individuals in the experimental group called the weather number and recorded the weather prediction before writing down their dreams. Those in the control group spent about 90 seconds (the time required to call the weather report) lying in bed before writing. Obtaining the weather report had a dramatic effect on dream recall: 33 percent of the students who called for the weather report recalled a dream, compared to 63 percent of those who did not call. Cohen concluded that engaging in various activities after we awaken interferes with our ability to remember our dreams.