Misconception 1: Sleep is time for the body in general and the brain specifically to shut down for rest

Sleep is an active process involving specific cues for its regulation. Although there are some modest decreases in metabolic rate, there is no evidence that any major organ or regulatory system in the body shuts down during sleep.³² Some brain activity, including delta waves, increases dramatically. Also, the endocrine system increases secretion of certain hormones during sleep, such as growth hormone and prolactin. In REM sleep, many parts of the brain are as active as at any time when awake.

Misconception 2: Getting just one hour less sleep per night than needed will not have any effect on daytime functioning

When daily sleep time is less than an individual needs, a "sleep debt" develops. Even relatively modest daily reductions in sleep time (for example, one hour) can accumulate across days to cause a sleep debt. If the debt becomes too great, it can lead to problem sleepiness. Although the individual may not realize his or her sleepiness, the sleep debt can have powerful effects on daytime performance, thinking, and mood.

The biological clock that times and controls a person's sleep/wake cycle will attempt to function according to a normal day/night schedule even when that person tries to change it.

Misconception 3: The body adjusts quickly to different sleep schedules

The biological clock that times and controls a person's sleep/wake cycle will attempt to function according to a normal day/night schedule even when that person tries to change it. Those who work night shifts naturally feel sleepy when nighttime comes. A similar feeling that occurs during travel is known as jet lag. (See Major Concepts, section 3.5, on pages 26–30.) This conflict, set up by trying to be active during the brain's biological nighttime, leads to a decrease in cognitive and motor skills. The biological clock can be reset, but only by appropriately timed cues and even then, by one to two hours per day at best.¹² Problems resulting from a mismatch of this type may be reduced by behaviors such as sleeping in a dark, quiet room, getting exposure to bright light at the right time, and altering eating and exercise patterns. Because humans function best when they sleep at night and act in the daytime, the task for a person who must be active at night is to retrain the biological clock (by light cues).

Misconception 4: People need less sleep as they grow older

Older people don't need less sleep, but they often get less sleep. That's because the ability to sleep for long periods of time and to get into the deep, restful stages of sleep decreases with age. Many older people have more fragile sleep and are more easily disturbed by light, noise, and pain than when younger. They are also more likely to have medical conditions that contribute to sleep problems.

Misconception 5: A "good night's sleep" can cure problems with excessive daytime sleepiness

Excessive daytime sleepiness can be associated with a sleep disorder or other medical condition. Sleep disorders, including sleep apnea (that is, absence of breathing during sleep), insomnia, and narcolepsy, may require behavioral, pharmacological, or even surgical intervention to relieve the symptoms.^{22, 24} Extra sleep may not eliminate daytime sleepiness that may be due to such disorders.

Sleep is a dynamic process

Sleep is not a passive event, but rather an active process involving characteristic physiological changes in the organs of the body. Scientists study sleep by measuring the electrical changes in the brain using electroencephalograms (EEGs). Typically, electrodes are placed on the scalp in a symmetrical pattern. The electrodes measure very small voltages that scientists think are caused by synchronized activity in very large numbers of synapses (nerve connections) in the brain's outer layers (cerebral cortex). EEG data are represented by curves that are classified according to their frequencies. The wavy lines of the EEG are called brain waves. An electrooculogram (EOG) uses electrodes on the skin near the eye to measure changes in voltage as the eye rotates in its socket. Scientists also measure the electrical activity associated with active muscles by using electromyograms (EMGs). In this technique, electrodes are placed on the skin overlaying a muscle. In humans, the electrodes are placed under the chin because muscles in this area demonstrate very dramatic changes during the various stages of sleep.

In practice, EEGs, EOGs, and EMGs are recorded simultaneously on continuously moving chart paper or digitized by a computer and displayed on a high-resolution monitor. This allows the relationships among the three measurements to be seen immediately. The patterns of activity in these three systems provide the basis for classifying the different types of sleep.

Figure 2

Placement of electrodes to determine EEG, EOG, and EMG.

Studying these events has led to the identification of two basic stages, or states, of sleep: non rapid eye movement (NREM) and rapid eye movement (REM). ^{15, 38}

Sleep is a highly organized sequence of events that follows a regular, cyclic program each night. Thus, the EEG, EMG, and EOG patterns change in predictable ways several times during a single sleep period. NREM sleep is divided into four stages according to the amplitude and frequency of brain wave activity. In general, the EEG pattern of NREM sleep is slower, often more regular, and usually of higher voltage than that of wakefulness. As sleep gets deeper, the brain waves get slower and have greater amplitude. NREM Stage 1 is very light sleep; NREM Stage 2 has special brain waves called sleep spindles and K complexes; NREM Stages 3 and 4 show increasingly more high-voltage slow waves. In NREM Stage 4, it is extremely hard to be awakened by external stimuli. The muscle activity of NREM sleep is low, but the muscles retain their ability to function. Eye movements normally do not occur during NREM sleep, except for very slow eye movements, usually at the beginning. The body's general physiology during these stages is fairly similar to the wake state. In this module, we will emphasize NREM sleep in general and not its individual substages.

The EEG recorded during REM sleep shows very fast and desynchronized activity that is more random than that recorded during NREM sleep. It actually looks similar to the EEG (low voltage with a faster mix of frequencies) from when we are awake. REM sleep is characterized by bursts of rapid eye movements. The eyes are not constantly moving, but they dart back and forth or up and down. They also stop for a while and then jerk back and forth again. Always, and just like waking eye movements, both eyes move together in the same direction. Some scientists believe that the eye movements of REM sleep relate to the visual images of dreams, but why they exist and what function they serve, if any, remain unknown. Additionally, while muscle tone is normal in NREM sleep, we are almost completely paralyzed in REM sleep. Although the muscles that move our bodies go limp, other important muscles continue to function in REM sleep. These include the heart, diaphragm, eye muscles, and smooth muscles such as those of the intestines and blood vessels. The paralysis of muscles in the arms and legs and under the chin show electrical silence in REM sleep. On an EMG, the recording produces a flat line. Small twitches can break through this paralysis and look like tiny blips on the flat line.

Sleep is a cyclical process. During sleep, people experience repeated cycles of NREM and REM sleep, beginning with an NREM phase. This cycle lasts approximately 90 to 110 minutes and is repeated four to six times per night. As the night progresses, however, the amount of deep NREM sleep decreases and the amount of REM sleep increases. Figure 3 graphically depicts the pattern of cycling we experience. The term ultradian rhythm (that is, rhythm occurring within a period of less than 24 hours) is used to describe this cycling through sleep stages.

Figure 3

Characteristic EEG, EOG, and EMG patterns for wakefulness, REM sleep, and NREM sleep. Each of the nine patterns was made over a period of about three seconds.

The chart in Figure 4 is called a hypnogram. Hypnograms were developed to summarize the voluminous chart recordings of electrical activities (EEG, EOG, and EMG) collected during a night's sleep. Hypnograms provide a simple way to display information originally collected on many feet of chart paper or stored as a large file on a computer.

Figure 4

. A typical hypnogram from a young, healthy adult. Light-gray areas represent non–rapid eye movement (NREM) sleep.

We can make several observations about the hypnogram in Figure 4. First, the periods of NREM and REM sleep alternate during the night. Second, the deepest stages of NREM sleep occur in the first part of the night. Third, the episodes of REM sleep are longer as the night progresses. This hypnogram also indicates two periods during the night when the individual awakened (at about six and seven hours into the night).

It is useful to distinguish between sleep and the state induced during general anesthesia or seen in people who are in a coma. While these latter individuals are often said to be "asleep," their conditions are not readily reversible (that is, they cannot be awakened by a strong stimulus), and they do not exhibit the same brain wave patterns characteristic of true sleep.

Physiological changes during sleep

Table 1 summarizes some basic physiological changes that occur in NREM and REM sleep.

Table 1

Comparison of Physiological Changes During NREM and REM Sleep.

The functions of many organ systems are also linked to the sleep cycle, as follows:

• Endocrine system. Most hormone secretion is controlled by the circadian clock or in response to physical events. Sleep is one of the events that modify the timing of secretion for certain hormones. Many hormones are secreted into the blood during sleep. For example, scientists believe that the release of growth hormone is related in part to repair processes that occur during sleep. Follicle stimulating hormone and luteinizing hormone, which are involved in maturational and reproductive processes, are among the hormones released during sleep. In fact, the sleep-dependent release of luteinizing hormone is thought to be the event that initiates puberty. Other hormones, such as thyroid-stimulating hormone, are released prior to sleep.
• Renal system. Kidney filtration, plasma flow, and the excretion of sodium, chloride, potassium, and calcium all are reduced during both NREM and REM sleep. These changes cause urine to be more concentrated during sleep.
• Alimentary activity. In a person with normal digestive function, gastric acid secretion is reduced during sleep. In those with an active ulcer, gastric acid secretion is actually increased. In addition, swallowing occurs less frequently.

Sleep and the brain

Sleep is actively generated in specific brain regions. These sites have been identified through studies involving electrical stimulation, damage to specific brain regions, or other techniques that identify sleep-inducing sites. The basal forebrain, including the hypothalamus, is an important region for controlling NREM sleep and may be the region keeping track of how long we have been awake and how large our sleep debt is. The brainstem region known as the pons is critical for initiating REM sleep. As depicted in Figure 5, during REM sleep, the pons sends signals to the visual nuclei of the thalamus and to the cerebral cortex (this region is responsible for most of our thought processes). The pons also sends signals to the spinal cord, causing the temporary paralysis that is characteristic of REM sleep. Other brain sites are also important in the sleep process. For example, the thalamus generates many of the brain rhythms in NREM sleep that we see as EEG patterns.

Figure 5

Pathways of brain activity during REM sleep.

Sleep patterns

Sleep patterns change during an individual's life. In fact, age affects sleep more than any other natural factor. Newborns sleep an average of 16 to 18 hours per day. By the time a child is three to five years old, total sleep time averages 10 to 12 hours, and then it further decreases to 7 to 8 hours per night by adulthood. One of the most prominent age-related changes in sleep is a reduction in the time spent in the deepest stages of NREM (Stages 3 and 4) from childhood through adulthood. In fact, this change is prominent during adolescence, when about 40 percent of this activity is lost and replaced by Stage 2 NREM sleep. In addition to these changes, the percentage of time spent in REM sleep also changes during development. Newborns may spend about 50 percent of their total sleep time in REM sleep. In fact, unlike older children and adults, infants fall asleep directly into REM sleep. Infant sleep cycles generally last only 50 to 60 minutes. By two years of age, REM sleep accounts for 20 to 25 percent of total sleep time, which remains relatively constant throughout the remainder of life.¹⁵ Young children have a high arousal threshold, which means they can sleep through loud noises, especially in the early part of the night. For example, one study showed that 10-year-olds were undisturbed by a noise as loud as the sound of a jet airplane taking off nearby.

Although most humans maintain REM sleep throughout life, brain disorders like Alzheimer's and Parkinson's are characterized by decreasing amounts of REM sleep as the diseases progress. Also, elderly individuals exhibit more variation in the duration and quality of sleep than do younger adults. Elderly people may also exhibit increased sleep fragmentation (arousals from sleep that occur as either short or more extended awakenings). Figure 6 depicts these developmental changes in sleep patterns.

Figure 6

Average sleep need (left graph) and percentage of REM sleep (right graph) at different ages.

Teenagers, on average, require about nine or more hours of sleep per night to be as alert as possible when awake.

Several issues are important to consider. First, individual sleep needs vary. For instance, eight hours of sleep per night appears to be optimal for most adults, although some may need more or less. Teenagers, on average, require about nine or more hours of sleep per night to be as alert as possible when awake. If sleep needs are not met, a progressive sleep debt occurs and eventually the body requires that the debt be paid. It does not appear that we are able to adapt to getting less sleep than our bodies require. Not getting enough sleep, while still allowing us to function in a seemingly normal manner, does impair motor and cognitive functions. Caffeine and other stimulants cannot substitute for sleep, but they do help to counteract some of the effects of sleep deprivation.

Biological clock

An internal biological clock regulates the timing for sleep in humans. The activity of this clock makes us sleepy at night and awake during the day. Our clock cycles with an approximately 24-hour period and is called a circadian clock (from the Latin roots circa = about and diem = day). In humans, this clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus in the brain (see Figure 7). ²¹ The SCN is actually a very small structure consisting of a pair of pinhead-size regions, each containing only about 10,000 neurons out of the brain's estimated 100 billion neurons.

Figure 7

The biological clock is located within the suprachiasmatic nucleus in the brain.

Biological clocks are genetically programmed physiological systems that allow organisms to live in harmony with natural rhythms, such as day/night cycles and the changing of seasons. The most important function of a biological clock is to regulate overt biological rhythms like the sleep/wake cycle. The biological clock is also involved in controlling seasonal reproductive cycles in some animals through its ability to track information about the changing lengths of daylight and darkness during a year.

Biological rhythms are of two general types. Exogenous rhythms are directly produced by an external influence, such as an environmental cue. They are not generated internally by the organism itself, and if the environmental cues are removed, the rhythm ceases. Endogenous rhythms, by contrast, are driven by an internal, self-sustaining biological clock rather than by anything external to the organism. Biological rhythms, such as oscillations in core body temperature, are endogenous. They are maintained even if environmental cues are removed.

Because the circadian clock in most humans has a natural day length of just over 24 hours, the clock must be entrained, or reset, to match the day length of the environmental photoperiod (that is, the light/dark, or day/night, cycle).

Because the circadian clock in most humans has a natural day length of just over 24 hours, the clock must be entrained, or reset, to match the day length of the environmental photoperiod (that is, the light/dark, or day/night, cycle). The cue that synchronizes the internal biological clock to the environmental cycle is light. Photoreceptors in the retina transmit light-dependent signals to the SCN. Interestingly, our usual visual system receptors, the rods and cones, are apparently not required for this photoreception.⁹ Special types of retinal ganglion cells are photoreceptive, project directly to the SCN, and appear to have all the properties required to provide the light signals for synchronizing the biological clock.³ At the SCN, the signal interacts with several genes that serve as "pacemakers."

Endogenous sleep rhythms can be depicted graphically. Figure 8 shows a day-by-day representation of one individual's sleep/wake cycle. The black lines indicate periods of sleep, and the gray lines indicate periods of wakefulness. The upper portion of the figure (days 1 through 9) represents this individual's normal sleep/wake cycle. Under these conditions, the individual is exposed to regularly timed exposure to alternating daylight and darkness, which has entrained this person's sleep/wake cycling to a period of 24 hours.

Figure 8

Entrainment of the biological clock. Black bars, asleep; gray bars, awake.

Contrast this upper portion of the figure with the middle portion (days 10 through 34). In the middle portion, this individual has been isolated from normal environmental cues like daylight, darkness, temperature variation, and noise variation. There are two important points to be derived from this portion of the figure. First, this individual's sleep/wake cycle continues to oscillate in the absence of external cues, stressing that this rhythm is endogenous, or built in. Second, in the absence of external cues to entrain circadian rhythms, this individual's clock cycles with its own natural, built-in rhythm that is just over 24 hours long. Consequently, without environmental cues, the individual goes to bed about one hour later each night. After 24 days, the individual is once again going to bed at midnight. The lower portion of Figure 8 depicts the change in the sleep/wake cycle after the individual is once again entrained to a 24-hour day containing the proper environmental cues.

Another interesting rhythm that is controlled by the biological clock is the cycle of body temperature, which is lowest in the biological night and rises in the biological daytime. This fluctuation persists even in the absence of sleep. Activity during the day and sleep during the night reinforce this cycle of changes in body temperature, as seen in Figure 9.

Figure 9

Body temperature in relation to the sleep cycle.

The release of melatonin, a hormone produced by the pineal gland, is controlled by the circadian clock in the SCN. Its levels rise during the night and decline at dawn in both nocturnal and diurnal species. Melatonin has been called the hormone of darkness because of this pattern. The SCN controls the timing of melatonin release; melatonin then feeds back on the SCN to regulate its activity. In mammals, for example, most of the brain receptors for melatonin are located in the SCN. Research has demonstrated that administering melatonin can produce shifts in circadian rhythms in a number of species including rats, sheep, lizards, birds, and humans. These effects are most clearly evident when melatonin is given in the absence of light input. Thus, for example, giving melatonin to blind people can help set their biological clocks. Melatonin is available as an over-the-counter nutritional supplement. Although claims are made that the supplement promotes sleep, the evidence for this is inconclusive. Potential side effects of long-term administration of melatonin remain unknown, and its unsupervised use by the general public is discouraged.

In addition to synchronizing these daily rhythms, biological clocks can affect rhythms that are longer than 24 hours, especially seasonal rhythms. Some vertebrates have reproductive systems that are sensitive to day length. These animals can sense changes in day length by the amount of melatonin secreted. The short days and long nights of winter turn off the reproductive systems of hamsters, while in sheep the opposite occurs. The high levels of melatonin that inhibit reproduction in hamsters stimulate the reproductive systems of sheep, so they breed in winter and give birth in the spring.

Biological clocks exist in a wide range of organisms, from cyanobacteria (blue-green algae) to humans. Clocks enable organisms to adapt to their surroundings. Although scientists currently believe that clocks arose through independent evolution and may use different clock proteins, they all share several regulatory characteristics. In particular, they are maintained by a biochemical process known as a negative feedback loop.

Much of what is known about clock regulation has come from studying the fruit fly Drosophila melanogaster, from which biological clock genes were first cloned. Two genes called period (per) and timeless (tim) were found to cycle with a 24-hour, or circadian, rhythm.^{8, 12} The genes are active early in the night and produce mRNA that is then translated into the proteins PER and TIM. These proteins begin to accumulate in the cytoplasm. After the proteins have reached high enough levels, PER protein binds to TIM protein, forming a complex that enters the cell's nucleus. In the nucleus, the PER-TIM complexes bind to the per and tim genes to suppress further transcription. This creates what is called a negative feedback loop. After a while, the PER and TIM proteins degrade, and transcription from the per and tim genes begins again.

This description of Drosophila's clock is a simplified one. Other genes have been identified that produce proteins involved with regulating the circadian clock. For example, the proteins CLOCK, CYCLE, and VRILLE are transcription factors that regulate expression of the per and tim genes. Other proteins, like the enzymes DOUBLE-TIME and SHAGGY, can alter the periodicity of the clock through chemical modification (phosphorylation) of PER and TIM. Mutations have been identified in clock genes that speed up, slow down, or eliminate the periodicity of the circadian clock in flies. Interestingly, similar genes and proteins have been identified in mammals, and studies indicate that the mammalian clock is regulated in much the same way as that of the fly.^{8, 10, 11, 33} Developmental changes in the circadian clock occur from infancy to childhood to adolescence, and further changes occur as adults age. Very little is known about specific genes and mediators responsible for the normal development of the circadian clock.

Figure 10

In the fruit fly, the biological clock is largely controlled by two genes called per and tim, whose expression cycles with an approximately 24-hour period. This cycling of gene expression is controlled by a process called a negative feedback loop.

Jet lag results from the inability of our circadian clock to make an immediate adjustment to the changes in light cues that come from a rapid change in time zone.

One negative consequence of our circadian cycle afflicts travelers who rapidly cross multiple time zones. Jet lag produces a number of unwanted effects including excessive sleepiness, poor sleep, loss of concentration, poor motor control, slowed reflexes, nausea, and irritability. Jet lag results from the inability of our circadian clock to make an immediate adjustment to the changes in light cues that an individual experiences when rapidly crossing time zones. After such travel, the body is in conflict. The biological clock carries the rhythm entrained by the original time zone, even though the clock is out of step with the cues in the new time zone. This conflict between external and internal clocks and signals is called desynchronization, and it affects more than just the sleep/wake cycle. All the rhythms are out of sync, and they take a number of days to re-entrain to the new time zone. Eastward travel generally causes more severe jet lag than westward travel, because traveling east requires that we shorten our day and adjust to time cues occurring earlier than our clock is used to. In general, the human circadian clock appears better able to adjust to a longer day than a shorter day. For example, it is easier for most people to adjust to the end of daylight savings time in the fall when we have one 25-hour day than to the start of daylight savings time in the spring, when we have a 23-hour day. Similarly, traveling from the West Coast of the United States to the East Coast produces a loss of three hours —a 21-hour day. Thus, travelers may find it difficult to sleep because of the three-hour difference between external cues and their internal clock. Likewise, travelers may find it difficult to awaken in the morning. We may try to go to sleep and wake up at our usual local times of, say, 11 p.m. and 7 a.m., but to our brain's biological clock, the times are 8 p.m. and 4 a.m. Other circadian rhythm problems include

• Monday morning blues. By staying up and sleeping in an hour or more later than usual on the weekends, we provide our biological clock different cues that push it toward a later nighttime phase. By keeping a late sleep schedule both weekend nights, our internal clock becomes two hours or more behind our usual weekday schedule. When the alarm rings at 6:30 a.m. on Monday, our body's internal clock is now set for 4:30 a.m. or earlier.
• Seasonal affective disorder (SAD). A change of seasons in autumn brings on both a loss of daylight savings time (fall back one hour) and a shortening of the daytime. As winter progresses, the day length becomes even shorter. During this season of short days and long nights, some individuals develop symptoms similar to jet lag but more severe. These symptoms include decreased appetite, loss of concentration and focus, lack of energy, feelings of depression and despair, and excessive sleepiness. Too little bright light reaching the biological clock in the SCN appears to bring on this recognized form of depression in susceptible individuals. Consequently, treatment often involves using light therapy.
• Shift work. Unlike some animals, humans are active during daylight hours. This pattern is called diurnal activity. Animals that are awake and active at night (for example, hamsters) have what is known as nocturnal activity. For humans and other diurnally active animals, light signals the time to awake, and sleep occurs during the dark. Modern society, however, requires that services and businesses be available 24 hours a day, so some individuals must work the night shift. These individuals no longer have synchrony between their internal clocks and external daylight and darkness signals, and they may experience mental and physical difficulties similar to jet lag and SAD.

Homeostasis and sleep

The relationship of circadian rhythms to sleep is relatively well understood. Continuing studies in genetics and molecular biology promise further advances in our knowledge of how the circadian clock works and how a succession of behavioral states adapt to changes in light/dark cycles. In addition to the circadian component, there is a fundamental regulatory process involved in programming sleep. Consider that the longer an individual remains awake, the stronger the desire and need to sleep become. This pressure to sleep defines the homeo-static component of sleep. The precise mechanism underlying the pressure that causes us to feel a need to sleep remains a mystery. What science does know is that the action of nerve-signaling molecules called neurotransmitters and of nerve cells (neurons) located in the brainstem and at the base of the brain determines whether we are asleep or awake. Additionally, there is recent evidence that the molecule adenosine (composed of the base adenine linked to the five-carbon sugar ribose) is an important sleepiness factor: it appears to "keep track" of lost sleep and may induce sleep. Interestingly, caffeine binds to and blocks the same cell receptors that recognize adenosine.^{21, 30} This suggests that caffeine disrupts sleep by binding to adenosine receptors and preventing adenosine from delivering its fatigue signal. The homeostatic regulation of sleep helps reinforce the circadian cycle. We usually sleep once daily because the homeostatic pressure to sleep is hard to resist after about 16 hours, and then while we sleep, our closed eyes block the light signals to the biological clock. See Figure 11.

Figure 11

Homeostatic regulation of sleep: the pressure to sleep grows stronger across the day as one stays awake and then dissipates when one sleeps at night (shaded area). Sleep pressure increases (dashed line) as one stays awake longer into the normal sleeping (more...)

Dreams

An intriguing occurrence during sleep is dreaming. Although reports of dreaming are most frequent and vivid when an individual is aroused from REM sleep, dreams do occur at sleep onset and during NREM sleep as well.²³ During an average night's sleep, about two hours are spent dreaming, mostly during REM sleep. Although some dreams are memorable because of their extraordinary or bizarre nature, other dreams reflect realistic experiences. Despite this realism, REM dreams are usually novel experiences, like a work of fiction, instead of a replay of actual events. Pre-sleep stimuli do not seem to affect dream content. In fact, the source of the content of any given dream is unknown. REM sleep and dreams are associated with each other, but they are not synonymous. While REM sleep is turned on and off by the pons (see section 3.3 Sleep and the brain, page 24), two areas in the cerebral hemispheres (areas far from the pons that control higher mental functions) regulate dreaming.

REM sleep and dreaming can be dissociated from one another, as seen after the administration of certain drugs or in cases of brain damage either to the pons (loss of REM sleep but not of dreaming) or to the frontal areas (no dreaming but REM sleep cycle unaffected). Consequently, REM sleep appears to be just one of the triggers for dreaming. Using scanning techniques that assess brain activity, scientists have determined which areas of the brain are active during REM sleep dreaming.^{4, 14} These areas are illustrated in Figure 12. Brain regions that are inactive during dreaming include those that regulate intelligence, conscious thought, and higher-order reasoning. Higher-order reasoning is that part of brain function responsible for processing experiences into memory and regulating vision while we are awake. The significance of dreaming to one's health and the meaning of dreams remain mysteries.

Figure 12

Areas of the brain active during REM sleep dreaming.

Scientists still do not fully understand the functions of sleep.

Functions of sleep

Animal studies have demonstrated that sleep is essential for survival. Consider studies that have been performed with laboratory rats. While these animals will normally live for two to three years, rats deprived of REM sleep survive an average of only five months. Rats deprived of all sleep survive only about three weeks.³² In humans, extreme sleep deprivation can cause an apparent state of paranoia and hallucinations in otherwise healthy individuals. However, despite identifying several physiological changes that occur in the brain and body during sleep, scientists still do not fully understand the functions of sleep. Many hypotheses have been advanced to explain the role of this necessary and natural behavior.³² The following examples highlight several of these theories:

Evolution of sleep

Sleep is ubiquitous among mammals, birds, and reptiles, although sleep patterns, habits, postures, and places of sleep vary greatly. Consider the following:

• Sleep patterns. Mammals generally alternate between NREM and REM sleep states in a cyclic fashion as described earlier, although the length of the sleep cycle and the percentage of time spent in NREM and REM states vary with the animal. Birds also have NREM/REM cycles, although each phase is very short (NREM sleep is about two and one-half minutes; REM sleep is about nine seconds). Additionally, birds do not lose muscle tone during REM sleep, a good thing for animals that sleep while standing or perching. Most scientific studies have failed to demonstrate REM sleep in reptiles. These findings have led some scientists to suggest that REM sleep may be a later evolutionary development related to warm-blooded animals.
• Sleep habits. Some mammals, such as humans, sleep primarily at night, while other mammals, such as rats, sleep primarily during the day. Furthermore, most (but not all) small mammals tend to sleep more than large ones (see Table 2 for examples). In some cases, animals have developed ways to sleep and concurrently satisfy critical life functions. These animals engage in unihemispheric sleep, in which one side of the brain sleeps while the other side is awake. This phenomenon is observed most notably in birds (like those that make long, transoceanic flights) and aquatic mammals (like dolphins and porpoises). Unihemispheric sleep allows aquatic mammals to sleep and continue to swim and surface to breathe. It allows animals to keep track of other group members and watch for predators. In fact, there is recent scientific evidence that mallard ducks can increase their use of unihemispheric sleep as the risk of predation increases.³¹
• Sleep postures. A wide variety of postures are seen: from sleeping curled up (dogs, cats, and many other animals), to standing (horses, birds), swimming (aquatic mammals, ducks), hanging upside down (bats), straddling a tree branch (leopard), and lying down (humans).
• Sleep places. Again, there is great variety, especially for mammals: burrows (rabbits), open spaces (lions), under water (hippopotami), nests (gorillas), and the comfort of one's own bed (humans).

Table 2

Representative Total Sleep Requirements for Various Species.

Sleep may also occur among lower life forms, such as fish and invertebrates, but it is hard to know because EEG patterns are not comparable to those of vertebrates. Consequently, investigating sleep in species other than mammals and birds has relied on the identification of specific behavioral characteristics of sleep: a quiet state, a typical species-specific sleep posture, an elevated arousal threshold (or reduced responsiveness to external stimuli), rapid waking due to moderately intense stimulation (that is, sleep is rapidly reversible), and a regulated response to sleep deprivation. Recent research demonstrates that even the fruit fly Drosophila melanogaster responds similarly to mammals when exposed to chemical agents that alter sleep patterns.^{13, 34}

Comparative studies have explored the evolution of sleep. Although REM sleep is thought to have evolved from NREM sleep, recent studies suggest that NREM and REM sleep may have diverged from a common precursor sleep state.

Sleep loss and wakefulness

About 30 to 40 percent of adults indicate some degree of sleep loss within any given year, and about 10 to 15 percent indicate that their sleep loss is chronic or severe.²⁰ In addition, millions of Americans experience problems sleeping because of undiagnosed sleep disorders or sleep deprivation. Adolescents and shift workers are at very high risk of problem sleepiness due to sleep deprivation and the desynchronized timing of sleep and wakefulness, respectively.

As outlined in section 3.5 Biological clock, sleep and wakefulness are linked in part to the activity of the circadian clock. Recent studies show that individual preferences for morning and evening activity may have a biological basis.³⁷ In addition, studies show that adolescents experience a delay in the circadian timing system that results in a tendency for them to stay up later and sleep in later.⁶ Loss of sleep creates an overwhelming and uncontrollable need to sleep and affects virtually all physiological functions. Sleep loss causes problems with memory and attention, complex thought, motor responses to stimuli, performance in school or on the job, and controlling emotions. Sleep loss may also alter thermoregulation and increase the risk for various physical and mental disorders.

Many adolescents are chronically sleep-deprived and hence at high risk of drowsy-driving crashes.

Sleep loss affects personal safety on the road. The National Highway Traffic Safety Administration has estimated that approximately 100,000 motor vehicle crashes each year result from a driver's drowsiness or fatigue while at the wheel.²⁸ Driving at night or in the early to mid afternoon increases the risk of a crash because those are times that our biological clocks make us sleepy. Drowsy driving impairs a driver's reaction time, vigilance, and ability to make sound judgments. Many adolescents are chronically sleep-deprived and hence at high risk of drowsy-driving crashes. In one large study of fall-asleep crashes, over 50 percent occurred with a driver 25 years old or younger.