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All animals sleep or engage in some rest activity that resembles sleep (Horne, 2006). Dolphins snooze while swimming, keeping one eye cracked open at all times; horses usually sleep standing up; and some birds appear to doze mid-flight (Siegel, 2005; U.S. Fish & Wildlife Service, 2006). There are animals that require loads of sleep—bats and opossums sleep 18 to 20 hours a day—and those that need barely any—elephants and giraffes get by on 3 or 4 hours (Siegel, 2005). Sleep needs vary greatly among people, ranging from as little as 4 hours a night to as long as 11 or more (Horne, 2006). But most of us require between 7 and 8 hours to stay mentally and physically healthy (Banks & Dinges, 2007). Do the math and that translates to about a third of the day, and therefore a third of your life. Clearly, sleep serves some important function, but what is it? And how does it relate to consciousness? How can sleep go so wrong, as happened for Matt? Before tackling these questions, let’s get a handle on the basics.

LO 4 Identify how circadian rhythm relates to sleep.

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CIRCADIAN RHYTHM Have you ever noticed that you often get sleepy in the middle of the afternoon? Even if you had a good sleep the night before, you inevitably begin feeling tired around 2:00 or 3:00 P.M.; it’s like clockwork. That’s because it is clockwork. Many things your body does, including sleep, are regulated by a biological clock. Body temperature rises during the day, reaching its maximum in the early evening. Hormones are secreted in a cyclical fashion. Growth hormone is released at night, and the stress hormone cortisol soars in the morning, reaching levels 10 to 20 times higher than at night (Wright, 2002). These are just a few of the body functions that follow predictable daily patterns, affecting our behaviors, alertness, and activity levels. Such patterns in our physiological functioning roughly follow the 24-hour cycle of daylight and darkness; they follow a circadian rhythm (sər-ˈkā-dē-ən).

In the circadian rhythm for sleep and wakefulness, there are two times when the desire for sleep hits hardest. The first is between 2:00 and 6:00 A.M., the same window of time when most car accidents caused by sleepiness occur (Horne, 2006). The second, less intense desire for sleep strikes midafternoon, around 2:30 P.M., when many college students have trouble keeping their eyes open in class (Mitler & Miller, 1996).

Not all biological rhythms are circadian. Some occur over longer time intervals (monthly menstruation), and others cycle much faster (90-minute sleep cycles, to be discussed shortly). Many animals migrate or hibernate during certain seasons and mate according to a yearly pattern. Even when deprived of cues like changing levels of sunlight, some animals continue to follow these cycles. Birds caged indoors, for example, exhibit mood and behavioral changes at the times of year when they would normally be migrating. Biological clocks are everywhere in nature, acting as day planners for organisms as basic as bacteria and slime mold (Wright, 2002).

SUPRACHIASMATIC NUCLEUS Where in the human body do these inner clocks and calendars dwell? Miniclocks are found in cells all over your body, but a master clock is nestled deep within the hypothalamus, a brain structure whose activities revolve around maintaining homeostasis, or balance, in the body’s systems. This master of clocks, known as the suprachiasmatic nucleus (SCN), actually consists of two clusters, each no bigger than an ant, totaling around 20,000 neurons (Forger & Peskin, 2003; Wright, 2002). The SCN plays a role in our circadian rhythm by communicating with other areas of the hypothalamus, which regulates daily patterns of hunger and temperature, and the reticular formation, which regulates alertness and sleepiness (Infographic 4.1).

Although tucked away in the recesses of the brain, the SCN knows the difference between day and night. That’s because it receives signals from a special type of light-sensing cells in the eye, called retinal ganglion cells. With the help of these informants, the clock adjusts to the light and dark cycling of the planet. When light beams upon the earth, your clock tells you to rise and shine, and when darkness hits, it urges you to bed. One way the SCN keeps you on schedule is by indirectly communicating with the pineal gland, a part of the endocrine system, to regulate the release of melatonin, a hormone that promotes sleep. In dark conditions, the clock commands the pineal gland to produce melatonin, making it easier to sleep. When light hits the eye, melatonin secretion slows down. So if you want to sleep, turn down the lights, and let your brain turn up the melatonin.

What would happen if you lived in a dark cave with no cell phones or computers to help you keep track of time? Would your body stay on a 24-hour cycle or get confused? Studies of people living in conditions with no indication of the time of day show that the internal clock continues to hum along at a slightly slower pace, eventually settling on a cycle that runs a little over 24 hours (Carskadon, Labayak, Acebo, & Seifer, 1999; Czeisler et al., 1999). But depriving the clock of its external light cues is generally not a good idea. Sleep–wake cycles can be disrupted, leading to exhaustion, irritability, impairment of memory, and other negative outcomes.

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LARKS AND OWLS Everyone has her own unique clock, which helps explain why some of us are “morning people” or so-called “larks,” and others are “night owls.” If you are a lark, you roll out of bed feeling energized and alert, get more accomplished early in the day, yet grow weary as the day drags on. One study characterized larks as preferring to go to bed before 11:00 P.M. and rising before 8:00 A.M. (Gale & Martyn, 1998). Owls, on the other hand, get up late and hit the sack late. If you slam the “snooze” button on your alarm clock five times every morning, shower with your eyes closed, and act like a grouch at breakfast, you’re probably an owl. But being an owl often means your energy level builds throughout the day, making it easy to stay up late posting to Instagram or reading your textbook. About 20% of us are true owls, 20% are genuine larks, and the rest fall somewhere in between (Horne, 2006).

JET LAG AND SHIFT WORK Whether you are a lark or an owl, your biological clock is likely to become confused when you travel across time zones. Your clock does not automatically reset to match the new time. The physical and mental consequences of this delayed adjustment, known as “jet lag,” may include difficulty concentrating, headaches, and gastrointestinal distress. Fortunately, the biological clock can readjust by about 1 or 2 hours each day, eventually falling into step with the new environmental schedule (Cunha & Stöppler, 2011). Jet lag is frustrating, but at least it’s only temporary.

Now imagine plodding through life with a case of jet lag you just can’t shake. This is the tough reality for some of the world’s shift workers—firefighters, nurses, miners, power plant operators, and other professionals who work while the rest of the world is snuggled under the covers. Shift workers represent about 20% of the workforce in the United States and other developed countries, or 1 in 5 people who are employed (Di Lorenzo et al., 2003). Some work rotating shifts, which means they are constantly going to bed and waking up at different times; others consistently work the overnight shift, so their sleep-wake cycles are permanently out-of-step with the light and dark cycles of the earth. Constantly fighting the clock takes a heavy toll on the mind and body. An irregular sleep schedule may lead to symptoms of insomnia, or difficulty falling asleep and sleeping soundly. Picture yourself coming off the night shift and arriving home at 7:00 A.M.: The sun is shining brightly, the birds are chirping, and the rest of the family is chatting over their cornflakes. This is not an ideal environment for sleep. Insomnia resulting from shift work can lead to decreased job productivity, depression, anxiety, diabetes, and other chronic diseases (Morin et al., 2006; Vgontzas et al., 2009). Shift workers also face an elevated risk of becoming overweight, and of developing stomach ulcers and heart disease (Di Lorenzo et al., 2003; Knutsson, 2003). In addition, an estimated 5% to 10% of shift workers have been diagnosed with circadian rhythm sleep–wake disorders, characterized by excessive sleepiness at work and insomnia at home (American Psychiatric Association, 2013).

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Dr. Lawrence J. Epstein of Harvard Medical School suggests ways in which night workers can minimize circadian disturbances and increase their productivity. Remember that light is the master clock’s most important external cue. Dr. Epstein suggests maximizing light exposure during work time and steering clear of it close to bedtime. Some night shifters don sunglasses on their way home, to block the morning sun, and head straight to bed in a quiet, dark room (Epstein & Mardon, 2007). Taking 20- to 30-minute power naps in the middle of a night shift can also help shift workers stay awake and alert (Harvard Medical School, 2007).

LO 5 Summarize the stages of sleep.

Have you ever watched someone sleeping? The person looks blissfully tranquil: body still, face relaxed, chest rising and falling like a lazy ocean wave. Don’t be fooled. Underneath the body’s quiet front is a very active brain, as revealed by an electroencephalogram (EEG), a device that picks up electrical signals from the brain and displays this information on a screen. If you could look at an EEG trace of your brain right this moment, you would probably see a series of tiny, short spikes in rapid-fire succession. These high-frequency brain waves are called beta waves, and they appear when you are solving a math problem, reading a book, or any time you are alert. Now let’s say you climb into bed, close your eyes, and relax. As you become more and more drowsy, the EEG would likely begin showing alpha waves, which are lower in frequency than beta waves (Cantero, Atienza, Salas, & Gómez, 1999). At some point, you drift into a different level of consciousness known as sleep.

NON-REM SLEEP A normal sleeper begins the night in non-rapid eye movement (non-REM), or nondreaming, sleep, which has four stages (Infographic 4.2, below). The first and lightest is Stage 1 sleep, also known as “light sleep.” During Stage 1, muscles go limp and body temperature starts to fall. The eyeballs may move gently beneath the lids. If you looked at an EEG of a person in Stage 1, you would likely see theta waves, which are lower in frequency than both alpha and beta waves. This is the type of sleep many people deny having. Example: Your friend begins to snooze while watching TV, so you poke her in the ribs and say, “Wake up!” but she swears she wasn’t asleep. It is also during this initial phase of sleep that hallucinations, or imaginary sensations, can occur. Do you ever see blotches of color or bizarre floating images as you drift off to sleep? Or perhaps you have felt a sensation of falling or swinging and then jerked your arms or legs in response? False perceptions that occur during the limbo between wakefulness and sleep are called hypnagogic (ˌhip-nə-ˈgäj-ik) hallucinations, and they are no cause for concern—in most cases. More on this when we return to Matt’s story.

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After a few minutes in Stage 1, you move on to the next phase of non-REM sleep, called Stage 2 sleep, which is slightly deeper than Stage 1, so you are harder to awaken. Theta waves continue showing up on the EEG, along with little bursts of electrical activity called sleep spindles and large waves called K-complexes appearing every 2 minutes or so. Researchers suspect sleep spindles are associated with memory consolidation and intelligence (Fogel & Smith, 2011). The exact function of K-complexes is up for debate: Some suggest they are the brain’s way of being ready to awaken when the need arises, while others believe they are the mechanism for remaining asleep in spite of disturbing stimuli (Colrain, 2005).

After passing through Stages 1 and 2, the sleeper descends into Stage 3, and then into an even deeper Stage 4, when it is most difficult to awaken. Both Stages 3 and 4 are known as slow-wave sleep, because they are characterized by tall, low-frequency delta waves. Stages 3 and 4 are really very similar, but Stage 4 contains a higher proportion of delta waves (delta waves are evident more than 50% of the time). Waking a person from slow-wave sleep is not easy. Most of us feel groggy, disoriented, and downright irritated when jarred from a slow-wave slumber. This is also the peak time for the secretion of growth hormone, which helps children to grow taller and stronger, and to build tissue (Awikunprasert & Sittiprapaporn, 2012).

REM SLEEP You don’t stay in deep sleep for the remainder of the night, however. After about 40 minutes of Stage 4 sleep, you work your way back through the lighter stages of sleep, from Stage 4, to Stage 3, to Stage 2, and finally to Stage 1. Then, instead of waking up, you enter a fifth stage known as rapid eye movement (REM) sleep. During REM, the eyes often dart around, even though they are closed (hence the name “rapid eye movement” sleep). The brain is very active, with EEG recordings showing faster and shorter waves similar to that of someone who is wide awake. Pulse and breathing rate fluctuate, and blood flow to the genitals increases, which explains why people frequently wake up in a state of sexual arousal. Another name for REM sleep is paradoxical sleep, because the sleeper appears to be quiet and resting, but the brain is full of electrical activity. People roused from REM sleep often report having vivid, illogical dreams. Thankfully, the brain has a way of preventing us from acting out our dreams. During REM sleep, certain neurons in the brainstem control the voluntary muscles, keeping most of the body still.

What would happen if the neurons responsible for disabling the muscles during REM sleep were destroyed or damaged? Researchers led by Michel Jouvet in France and Adrian Morrison in the United States found the answer to that question in the 1960s and 1970s. Both teams showed that severing these neurons in the brains of cats caused them to act out their kitty dreams. Not only did the sleeping felines stand up; they arched their backs in fury, groomed and licked themselves, and hunted imaginary mice (Jouvet, 1979; Sastre & Jouvet, 1979).

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SLEEP ARCHITECTURE Congratulations. You have just completed one sleep cycle, working your way through Stages 1, 2, 3, and 4 of non-REM sleep and ending with a dream-packed episode of REM. Each of these cycles lasts about 90 minutes, and the average adult sleeper loops through five of them per night. The composition of these 90-minute sleep cycles changes during the night. During the first two cycles, a considerable amount of time is devoted to the deep sleep Stages 3 and 4. Halfway through the night, however, Stages 3 and 4 vanish. Meanwhile, the REM periods become progressively longer, with the first REM episode lasting only 5 to 10 minutes, and the final one lasting nearly a half-hour (Siegel, 2005). Therefore, we pack in most of our restorative sleep early in the night and most of the dreaming toward the end; and the sleep stage we spend the most time in—nearly half the night—is Stage 2 (Epstein & Mardon, 2007).

As we age, the makeup of our sleep cycles, or sleep architecture, changes. Older people spend less time in REM sleep and the deeply refreshing stages of non-REM sleep (3 and 4). Instead, they experience longer periods of light sleep (Stages 1 and 2), which can be interrupted easily by noises and movements (Ohayon, Carskadon, Guilleminault, & Vitiello, 2004). Could this be the reason many older people complain of sleeping poorly, waking up often, and feeling drowsy during the day? Not all elderly people have trouble sleeping, of course. Like most everything in life, sleep patterns vary considerably from one individual to the next.

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LO 6 Recognize various sleep disorders and their symptoms.

CATAPLEXY And that wasn’t all. Matt developed another debilitating symptom of narcolepsy: cataplexy, an abrupt loss of strength or muscle tone that occurs when a person is awake. During a severe cataplectic attack, some muscles go limp, and the body may collapse slowly to the floor like a rag doll. One moment Matt would be standing in the hallway laughing with friends; the next he was splayed on the floor unable to move a muscle. “It was like a tree being cut down [and] tipping over,” he recalls. Cataplexy attacks come on suddenly, usually during periods of emotional excitement (American Psychiatric Association, 2013). The effects usually wear off after several seconds, but severe attacks can render a person immobilized for minutes.

Cataplexy may completely disable the body, but it produces no loss in consciousness. Even during the worst attack, Matt remained completely aware of himself and his surroundings. He could hear people talking about him; sometimes they snickered in amusement. “Kids can be cruel,” Matt says. By junior year, Matt was having 60 to 100 attacks a day.

SLEEP PARALYSIS AND HYPNAGOGIC HALLUCINATIONS Matt also developed two other common narcolepsy symptoms: sleep paralysis and hypnagogic hallucinations. Sleep paralysis is a temporary paralysis that strikes just before falling asleep or upon waking (American Psychiatric Association, 2013). Recall that the body becomes paralyzed during REM sleep, but sometimes this paralysis sets in prematurely or fails to turn off on time. Picture yourself lying in bed, awake and fully aware yet unable to roll over, climb out of bed, or even wiggle a toe. You want to scream for help, but your lips won’t budge. Sleep paralysis is a common symptom of narcolepsy, but it can also strike ordinary sleepers. One study found that nearly a third of college students had experienced sleep paralysis at least once in their lives (Cheyne, Newby-Clark, & Rueffer, 1999). Episodes usually last a few seconds, but some go on for several minutes—a terrifying experience for most people.

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Sleep paralysis may seem scary, but now imagine seeing bloodthirsty vampires standing at the foot of your bed just as you are about to fall asleep. Earlier we discussed the hypnagogic hallucinations people can experience during Stage 1 sleep (seeing strange images, for example). But not all hypnagogic hallucinations involve harmless blobs. They can also be realistic visions of axe murderers or space aliens trying to abduct you (McNally & Clancy, 2005). Matt had a recurring hallucination of a man with a butcher knife racing through his doorway, jumping onto his bed, and stabbing him in the chest. Upon awakening, Matt would often quiz his mother with questions like, “When is my birthday?” or “What is your license plate number?” He wanted to verify she was real, not just another character in his dream. Like sleep paralysis, vivid hypnagogic hallucinations can occur in people without narcolepsy, too. Shift work, insomnia, and sleeping faceup are all factors that appear to heighten one’s risk (Cheyne, 2002; McNally & Clancy, 2005).

BATTLING NARCOLEPSY Throughout junior year, Matt took various medications to control his narcolepsy, but his symptoms persisted. Narcolepsy was beginning to interfere with virtually every aspect of his life. At the beginning of high school, Matt had a 4.0 grade point average; now he was working twice as hard and earning lower grades. Playing sports had become a major health hazard because his cataplexy struck wherever and whenever, without notice. If he collapsed while sprinting down the soccer field or diving for a basketball, he might twist an ankle, break an arm, or worse. It was during this time that Matt realized who his true friends were. “The people that stuck with me [then] are still my close friends now,” he says. Matt’s loyal buddies learned to recognize the warning signs of his cataplexy (for example, when he suddenly stands still and closes his eyes) and did everything possible to keep him safe, grabbing hold of his body and slowly lowering him to the ground. His buddies had his back—literally.

Approximately 1 in 2,500 people suffers from narcolepsy (Ohayon, 2011). It is believed to result from a failure of the brain to properly regulate sleep patterns. Normally, the boundaries separating sleep and wakefulness are relatively clear—you are awake, in REM sleep, or in non-REM sleep. With narcolepsy, the lines separating these different realms of consciousness fade, allowing sleep to spill into periods of wakefulness. The loss of muscle tone during cataplexy, sleep paralysis, and dreamlike hypnagogic hallucinations may be explained by occurrences of REM sleep in the midst of wakefulness (Attarian, Schenck, & Mahowald, 2000). In other words, REM sleep occurs in the wrong place, at the wrong time (see a summary of this and other sleep disturbances in Table 4.1).

REM SLEEP BEHAVIOR DISORDER Problems with REM regulation can also lead to other sleep disturbances, including REM sleep behavior disorder. The defining characteristics of this disorder include “repeated episodes of arousal often associated with vocalizations and/or complex motor behaviors arising from REM sleep” (American Psychiatric Association, 2013, p. 408). People with REM sleep behavior disorder are much like the cats in Morrison’s and Jouvet’s experiments; something has gone awry with the brainstem mechanism responsible for paralyzing their bodies during REM sleep, so they are able to move around and act out their dreams (Schenck & Mahowald, 2002). This is not a good thing, since the dreams of people with REM sleep behavior disorder tend to be unusually violent and action-packed, involving fights with wild animals and other attackers (Fantini, Corona, Clerici, & Ferini-Strambi, 2005). According to some research, up to 65% of REM sleep behavior disorder sufferers have injured either themselves or their bedmates at one point or another. Scrapes, cuts, and bruises are common, and traumatic brain injuries have also been reported (American Psychiatric Association, 2013; Aurora et al., 2010). REM sleep behavior disorder primarily affects older men (age 50 and up) and frequently foreshadows the development of serious neurodegenerative disorders—conditions such as Parkinson’s disease and dementia that are associated with the gradual decline and death of neurons (Boeve et al., 2007; Fantini et al., 2005; Postuma et al., 2009; Schenck & Mahowald, 2002). But, women and younger people are diagnosed with this disorder as well (American Psychiatric Association, 2013).

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BREATHING-RELATED SLEEP DISORDERS There are several breathing-related sleep disorders, but the most common is obstructive sleep apnea hypopnea (hī-pop-ˈnē-ə), characterized by a complete absence of air flow (apnea) or reduced air flow (hypopnea). During normal sleep, the airway remains open, allowing air to flow in and out of the lungs. With obstructive sleep apnea hypopnea, the upper throat muscles go limp, allowing the upper airway to close shut (American Psychiatric Association, 2013). Breathing stops for 10 seconds or more, causing blood oxygen levels to drop (Chung & Elsaid, 2009). The brain responds by commanding the body to wake up and breathe! The sleeper awakes and gasps for air, sometimes with a noisy nasal sound, and then drifts back to sleep. This process can repeat itself several hundred times per night, preventing a person from experiencing the deep stages of sleep so crucial for feeling reenergized in the morning. Most people have no memory of the repeated awakenings and wonder why they feel so exhausted during the day; they are completely unaware that they suffer from this serious sleep disturbance. Obstructive sleep apnea hypopnea is more common among men than women and is more prevalent in the obese, and in women after menopause. This condition is linked to increased risk of death in the elderly, traffic accidents, and reduced quality of life, as well as elevated blood pressure, which increases the risk of cardiovascular disease (American Psychiatric Association, 2013).

INSOMNIA The most prevalent sleep disorder is insomnia, which is characterized by an inability to fall asleep or stay asleep. People with insomnia often report that the quantity or quality of their sleep is not good. They may complain of waking up in the middle of the night or arising too early, and not being able to fall back asleep. Sleepiness during the day and difficulties with cognitive tasks are also reported (American Psychiatric Association, 2013). About a third of adults experience some symptoms of insomnia, and 6% to 10% suffer from insomnia disorder (American Psychiatric Association, 2013; Mai & Buysse, 2008; Roth, 2007). Insomnia symptoms can be related to many factors, including the stress of a new job, college studies, depression, anxiety, jet lag, aging, and drug use.

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OTHER SLEEP DISTURBANCES A common sleep disturbance that can occur during non-REM sleep (typically Stages 3 and 4) is sleepwalking. A quarter of all children will experience at least one sleepwalking incident, and it seems to run in families (Licis, Desruisseau, Yamada, Duntley, & Gurnett, 2011). Here are some ways to spot a sleepwalker: Her face is expressionless; her eyes are open; and she may sit up in bed, walk around in confusion, or speak gibberish. (The garbled speech of sleepwalking is different from sleep talking, which can occur in either REM or non-REM sleep, but is not considered a sleep disturbance.) Sleepwalkers may have “limited recall” of the event upon awakening (American Psychiatric Association, 2013). They are capable of accomplishing a variety of tasks such as opening doors, going to the bathroom, and getting dressed, all of which they are likely to forget by morning. Most sleepwalking episodes are not related to dreaming, and contrary to urban myth, awakening a sleepwalker will not cause sudden death or injury. What’s dangerous is leaving the front door unlocked and the car keys in the ignition, as sleepwalkers have been known to wander into the streets and even attempt driving (American Psychiatric Association, 2013).

Sleep terrors are non-REM sleep disturbances primarily affecting children. A child experiencing a night terror may sit up in bed, stare fearfully at nothing, and scream. Parents may find the child crying hysterically, breathing rapidly, and sweating. No matter what the parents say or do, the child remains inconsolable. Fortunately, sleep terrors only last a few minutes, and most children outgrow them. Sleep terrors are often worse for parents than they are for the child, who generally won’t remember the episode the next day (American Psychiatric Association, 2013).

Nightmares are frightening dreams that occur in REM sleep. They affect people of all ages. And unlike night terrors, nightmares can often be recalled in vivid detail. Because nightmares usually occur during REM sleep, they are generally not acted out (American Psychiatric Association, 2013).

Matt’s worst struggle with narcolepsy stretched through the last two years of high school. During this time, he was averaging 20 to 30 naps a day. You might think that someone who falls asleep so often would at least feel well rested while awake. This was not the case. Matt had trouble sleeping at night, and it was taking a heavy toll on his ability to think clearly. He remembers nodding off at the wheel a few times but continuing to drive, reassuring himself that everything was fine. He forgot about homework assignments and simple things people told him. Matt was experiencing two of the most common symptoms of sleep deprivation: impaired judgment and lapses in memory (Goel, Rao, Durmer, & Dinges, 2009).

Let’s face it. No one can function optimally without a good night’s sleep. But the expression “good night’s sleep” means something quite different from one person to the next. Newborns sleep anywhere from 10.5 to 18 hours per day, toddlers 11 to 14 hours, school-aged children 9 to 11 hours, and teens 8 to 10 hours (National Sleep Foundation, 2015a, 2015c). The average adult needs between 7 and 8 hours to feel restored, though some (including Madonna and Jay Leno) claim they get by on just 4 (Breus, 2009, May 6). People who average less than 4 or more than 11 hours, otherwise known as “extreme sleepers,” are very rare (Horne, 2006).

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SLEEP DEPRIVATION What happens to animals when they don’t sleep at all? Laboratory studies show that sleep deprivation kills rats faster than starvation (Rechtschaffen & Bergmann, 1995; Siegel, 2005). Curtailing sleep in humans leads to rapid deterioration of mental and physical well-being. Stay up all night for 48 hours and you can expect your memory, attention, reaction time, and decision making to suffer noticeably (Goel et al., 2009; Lim & Dinges, 2010). Sleepy people find it especially challenging to accomplish tasks that are monotonous and boring; those deprived of sleep have trouble focusing on a single activity, like keeping their eyes on the road while driving (Lim & Dinges, 2010). Using driving simulators and tests to measure alertness, hand-eye coordination, and other factors, researchers report that getting behind the wheel while sleepy is similar to driving drunk. Staying awake for just 17 to 19 consecutive hours (which many of us with demanding jobs, children, and social lives do regularly) produces the same effect as having a blood alcohol content (BAC) of 0.05%, the legal limit in many countries (Williamson & Feyer, 2000). Sleep loss also makes you more prone to microsleeps, or uncontrollable mini-naps lasting several seconds—enough time to miss a traffic light turning red. Staying awake for several days at a time (11 days is the current world record, based on experimental data; Gillin, 2002, March 25) produces a host of disabling effects, including fragmented speech, cognitive deficits, mood swings, and hallucinations (Gulevich, Dement, & Johnson, 1966).

A more chronic form of sleep deprivation results from insufficient sleep night-upon-night for weeks, months, or years. People in this category are less likely than their well-rested peers to exercise, eat healthy foods, have sex, and attend family events (National Sleep Foundation, 2009). They also face a greater risk for heart disease, diabetes, and weight gain (Sigurdson & Ayas, 2007), and have decreased immune system responses and slower reaction times (Besedovsky, Lange, & Born, 2012; Orzeł-Gryglewska, 2010). Many researchers suspect the obesity epidemic currently plaguing Western countries is linked to chronic sleep deprivation. Skimping on sleep appears to disrupt appetite-regulating hormones, which may lead to excessive hunger and overeating (Willyard, 2008).

REM DEPRIVATION So far we have only covered sleep loss in general, but remember there are two types of sleep: REM and non-REM. What happens if only one of these is compromised? Studies show that depriving people of Stages 3 and 4 sleep leads to physical symptoms such as fatigue and increased sensitivity to pain (Roehrs, Hyde, Blaisdell, Greenwald, & Roth, 2006). Preliminary research suggests depriving people of REM sleep in particular can cause emotional overreactions to threatening situations (Rosales-Lagarde et al., 2012). REM deprivation can also lead to REM rebound, an increased amount of time spent in REM sleep when one finally gets an opportunity to sleep in peace.

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WHY DO WE SLEEP? The exact purpose of sleep has yet to be identified. Drawing from sleep deprivation studies and other types of experiments, researchers have constructed various theories to explain why we spend so much time sleeping (Table 4.2). Here are three of the major ones:

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Whatever the purpose of sleep, there is no denying its importance. After a couple of sleepless nights, we are grumpy, clumsy, and unable to think straight. Although we may appreciate the value of sleep, we don’t always practice the best sleep habits—or know what they are. Read on to discover some behaviors and assumptions you should avoid.

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Question 1

1. circadian rhythms.

2. beta waves.

3. K-complexes.

4. retinal ganglion cells.

Question 2

1. Stage 1

2. Stage 2

3. Stage 3

4. Stage 4

Question 3

Question 4