Do you ever feel sleepy or "zone out" during the
day? Do you find it hard to wake up on Monday mornings? If so, you are familiar with the
powerful need for sleep. However, you may not realize that sleep is as essential for your
well-being as food and water.
Sleep: A Dynamic
Until the 1950s, most people thought of sleep as a
passive, dormant part of our daily lives. We now know that our brains are very active
during sleep. Moreover, sleep affects our daily functioning and our physical and mental
health in many ways that we are just beginning to understand.
Nerve-signaling chemicals called neurotransmitters
control whether we are asleep or awake by acting on different groups of nerve cells, or
neurons, in the brain. Neurons in the brainstem, which connects the brain with the spinal
cord, produce neurotransmitters such as serotonin and norepinephrine that keep some parts
of the brain active while we are awake. Other neurons at the base of the brain begin
signaling when we fall asleep. These neurons appear to "switch off" the signals
that keep us awake. Research also suggests that a chemical called adenosine builds up in
our blood while we are awake and causes drowsiness. This chemical gradually breaks down
while we sleep.
During sleep, we usually pass through five phases of
sleep: stages 1, 2, 3, 4, and REM (rapid eye movement) sleep. These stages progress
in a cycle from stage 1 to REM sleep, then the cycle starts over again with stage 1 (see
figure 1). We spend almost 50 percent of our
total sleep time in stage 2 sleep, about 20 percent in REM sleep, and the remaining 30
percent in the other stages. Infants, by contrast, spend about half of their sleep time in
During stage 1, which is light sleep, we drift in and out
of sleep and can be awakened easily. Our eyes move very slowly and muscle activity slows.
People awakened from stage 1 sleep often remember fragmented visual images. Many also
experience sudden muscle contractions called hypnic myoclonia, often preceded by a
sensation of starting to fall. These sudden movements are similar to the "jump"
we make when startled. When we enter stage 2 sleep, our eye movements stop and our brain
waves (fluctuations of electrical activity that can be measured by electrodes) become
slower, with occasional bursts of rapid waves called sleep spindles. In stage 3,
extremely slow brain waves called delta waves begin to appear, interspersed with
smaller, faster waves. By stage 4, the brain produces delta waves almost exclusively. It
is very difficult to wake someone during stages 3 and 4, which together are called deep
sleep. There is no eye movement or muscle activity. People awakened during deep sleep
do not adjust immediately and often feel groggy and disoriented for several minutes after
they wake up. Some children experience bedwetting, night terrors, or sleepwalking during
When we switch into REM sleep, our breathing becomes more
rapid, irregular, and shallow, our eyes jerk rapidly in various directions, and our limb
muscles become temporarily paralyzed. Our heart rate increases, our blood pressure rises,
and males develop penile erections. When people awaken during REM sleep, they often
describe bizarre and illogical tales ó dreams.
The first REM sleep period usually occurs about 70 to 90
minutes after we fall asleep. A complete sleep cycle takes 90 to 110 minutes on average.
The first sleep cycles each night contain relatively short REM periods and long periods of
deep sleep. As the night progresses, REM sleep periods increase in length while deep sleep
decreases. By morning, people spend nearly all their sleep time in stages 1, 2, and REM.
People awakened after sleeping more than a few minutes are
usually unable to recall the last few minutes before they fell asleep. This sleep-related
form of amnesia is the reason people often forget telephone calls or conversations
theyíve had in the middle of the night. It also explains why we often do not remember
our alarms ringing in the morning if we go right back to sleep after turning them off.
Since sleep and wakefulness are influenced by different
neurotransmitter signals in the brain, foods and medicines that change the balance of
these signals affect whether we feel alert or drowsy and how well we sleep. Caffeinated
drinks such as coffee and drugs such as diet pills and decongestants stimulate some parts
of the brain and can cause insomnia, or an inability to sleep. Many antidepressants
suppress REM sleep. Heavy smokers often sleep very lightly and have reduced amounts of REM
sleep. They also tend to wake up after 3 or 4 hours of sleep due to nicotine withdrawal.
Many people who suffer from insomnia try to solve the problem with alcohol ó the
so-called night cap. While alcohol does help people fall into light sleep, it also robs
them of REM and the deeper, more restorative stages of sleep. Instead, it keeps them in
the lighter stages of sleep, from which they can be awakened easily.
People lose some of the ability to regulate their body
temperature during REM, so abnormally hot or cold temperatures in the environment can
disrupt this stage of sleep. If our REM sleep is disrupted one night, our bodies
donít follow the normal sleep cycle progression the next time we doze off. Instead,
we often slip directly into REM sleep and go through extended periods of REM until we
"catch up" on this stage of sleep.
People who are under anesthesia or in a coma are often
said to be asleep. However, people in these conditions cannot be awakened and do not
produce the complex, active brain wave patterns seen in normal sleep. Instead, their brain
waves are very slow and weak, sometimes all but undetectable.
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How Much Sleep Do We
The amount of sleep each person needs depends on many
factors, including age. Infants generally require about 16 hours a day, while teenagers
need about 9 hours on average. For most adults, 7 to 8 hours a night appears to be the
best amount of sleep, although some people may need as few as 5 hours or as many as 10
hours of sleep each day. Women in the first 3 months of pregnancy often need several more
hours of sleep than usual. The amount of sleep a person needs also increases if he or she
has been deprived of sleep in previous days. Getting too little sleep creates a
"sleep debt," which is much like being overdrawn at a bank. Eventually, your
body will demand that the debt be repaid. We donít seem to adapt to getting less
sleep than we need; while we may get used to a sleep-depriving schedule, our judgment,
reaction time, and other functions are still impaired.
People tend to sleep more lightly and for shorter time
spans as they get older, although they generally need about the same amount of sleep as
they needed in early adulthood. About half of all people over 65 have frequent sleeping
problems, such as insomnia, and deep sleep stages in many elderly people often become very
short or stop completely. This change may be a normal part of aging, or it may result from
medical problems that are common in elderly people and from the medications and other
treatments for those problems.
Experts say that if you feel drowsy during the day, even
during boring activities, you havenít had enough sleep. If you routinely fall asleep
within 5 minutes of lying down, you probably have severe sleep deprivation, possibly even
a sleep disorder. Microsleeps, or very brief episodes of sleep in an otherwise
awake person, are another mark of sleep deprivation. In many cases, people are not aware
that they are experiencing microsleeps. The widespread practice of "burning the
candle at both ends" in western industrialized societies has created so much sleep
deprivation that what is really abnormal sleepiness is now almost the norm.
Many studies make it clear that sleep deprivation is
dangerous. Sleep-deprived people who are tested by using a driving simulator or by
performing a hand-eye coordination task perform as badly as or worse than those who are
intoxicated. Sleep deprivation also magnifies alcoholís effects on the body, so a
fatigued person who drinks will become much more impaired than someone who is well-rested.
Driver fatigue is responsible for an estimated 100,000 motor vehicle accidents and 1500
deaths each year, according to the National Highway Traffic Safety Administration. Since
drowsiness is the brainís last step before falling asleep, driving while drowsy can
Ė and often does Ė lead to disaster. Caffeine and other stimulants cannot
overcome the effects of severe sleep deprivation. The National Sleep Foundation says that
if you have trouble keeping your eyes focused, if you canít stop yawning, or if you
canít remember driving the last few miles, you are probably too drowsy to drive
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What Does Sleep Do
Although scientists are still trying to learn exactly
why people need sleep, animal studies show that sleep is necessary for survival. For
example, while rats normally live for two to three years, those deprived of REM sleep
survive only about 5 weeks on average, and rats deprived of all sleep stages live only
about 3 weeks. Sleep-deprived rats also develop abnormally low body temperatures and sores
on their tail and paws. The sores may develop because the ratsí immune systems become
impaired. Some studies suggest that sleep deprivation affects the immune system in
Sleep appears necessary for our nervous systems to work
properly. Too little sleep leaves us drowsy and unable to concentrate the next day. It
also leads to impaired memory and physical performance and reduced ability to carry out
math calculations. If sleep deprivation continues, hallucinations and mood swings may
develop. Some experts believe sleep gives neurons used while we are awake a chance to shut
down and repair themselves. Without sleep, neurons may become so depleted in energy or so
polluted with byproducts of normal cellular activities that they begin to malfunction.
Sleep also may give the brain a chance to exercise important neuronal connections that
might otherwise deteriorate from lack of activity.
Deep sleep coincides with the release of growth hormone in
children and young adults. Many of the bodyís cells also show increased production
and reduced breakdown of proteins during deep sleep. Since proteins are the building
blocks needed for cell growth and for repair of damage from factors like stress and
ultraviolet rays, deep sleep may truly be "beauty sleep." Activity in parts of
the brain that control emotions, decision-making processes, and social interactions is
drastically reduced during deep sleep, suggesting that this type of sleep may help people
maintain optimal emotional and social functioning while they are awake. A study in rats
also showed that certain nerve-signaling patterns which the rats generated during the day
were repeated during deep sleep. This pattern repetition may help encode memories and
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Dreaming and REM Sleep
We typically spend more than 2 hours each night dreaming.
Scientists do not know much about how or why we dream. Sigmund Freud, who greatly
influenced the field of psychology, believed dreaming was a "safety valve" for
unconscious desires. Only after 1953, when researchers first described REM in sleeping
infants, did scientists begin to carefully study sleep and dreaming. They soon realized
that the strange, illogical experiences we call dreams almost always occur during REM
sleep. While most mammals and birds show signs of REM sleep, reptiles and other
cold-blooded animals do not.
REM sleep begins with signals from an area at the base of
the brain called the pons (see figure
2). These signals travel to a brain region called the thalamus, which
relays them to the cerebral cortex ó the outer layer of the brain that is
responsible for learning, thinking, and organizing information. The pons also sends
signals that shut off neurons in the spinal cord, causing temporary paralysis of the limb
muscles. If something interferes with this paralysis, people will begin to physically
"act out" their dreams ó a rare, dangerous problem called REM sleep
behavior disorder. A person dreaming about a ball game, for example, may run headlong
into furniture or blindly strike someone sleeping nearby while trying to catch a ball in
REM sleep stimulates the brain regions used in learning.
This may be important for normal brain development during infancy, which would explain why
infants spend much more time in REM sleep than adults (see Sleep: A Dynamic Activity).
Like deep sleep, REM sleep is associated with increased production of proteins. One study
found that REM sleep affects learning of certain mental skills. People taught a skill and
then deprived of non-REM sleep could recall what they had learned after sleeping, while
people deprived of REM sleep could not.
Some scientists believe dreams are the cortexís
attempt to find meaning in the random signals that it receives during REM sleep. The
cortex is the part of the brain that interprets and organizes information from the
environment during consciousness. It may be that, given random signals from the pons
during REM sleep, the cortex tries to interpret these signals as well, creating a
"story" out of fragmented brain activity.
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Sleep and Circadian
Circadian rhythms are regular changes in mental and
physical characteristics that occur in the course of a day (circadian is Latin for
"around a day"). Most circadian rhythms are controlled by the bodyís
biological "clock." This clock, called the suprachiasmatic nucleus or SCN
(see figure 2), is actually a pair
of pinhead-sized brain structures that together contain about 20,000 neurons. The SCN
rests in a part of the brain called the hypothalamus, just above the point where
the optic nerves cross. Light that reaches photoreceptors in the retina (a tissue
at the back of the eye) creates signals that travel along the optic nerve to the SCN.
Signals from the SCN travel to several brain regions,
including the pineal gland, which responds to light-induced signals by switching
off production of the hormone melatonin. The bodyís level of melatonin normally
increases after darkness falls, making people feel drowsy. The SCN also governs functions
that are synchronized with the sleep/wake cycle, including body temperature, hormone
secretion, urine production, and changes in blood pressure.
By depriving people of light and other external time cues,
scientists have learned that most peopleís biological clocks work on a 25-hour cycle
rather than a 24-hour one. But because sunlight or other bright lights can reset the SCN,
our biological cycles normally follow the 24-hour cycle of the sun, rather than our innate
cycle. Circadian rhythms can be affected to some degree by almost any kind of external
time cue, such as the beeping of your alarm clock, the clatter of a garbage truck, or the
timing of your meals. Scientists call external time cues zeitgebers (German for
When travelers pass from one time zone to another, they
suffer from disrupted circadian rhythms, an uncomfortable feeling known as jet lag.
For instance, if you travel from California to New York, you "lose" 3 hours
according to your bodyís clock. You will feel tired when the alarm rings at 8 a.m.
the next morning because, according to your bodyís clock, it is still 5 a.m. It
usually takes several days for your body's cycles to adjust to the new time.
To reduce the effects of jet lag, some doctors try to
manipulate the biological clock with a technique called light therapy. They expose people
to special lights, many times brighter than ordinary household light, for several hours
near the time the subjects want to wake up. This helps them reset their biological clocks
and adjust to a new time zone.
Symptoms much like jet lag are common in people who work
nights or who perform shift work. Because these peopleís work schedules are at odds
with powerful sleep-regulating cues like sunlight, they often become uncontrollably drowsy
during work, and they may suffer insomnia or other problems when they try to sleep. Shift
workers have an increased risk of heart problems, digestive disturbances, and emotional
and mental problems, all of which may be related to their sleeping problems. The number
and severity of workplace accidents also tend to increase during the night shift. Major
industrial accidents attributed partly to errors made by fatigued night-shift workers
include the Exxon Valdez oil spill and the Three Mile Island and Chernobyl nuclear power
plant accidents. One study also found that medical interns working on the night shift are
twice as likely as others to misinterpret hospital test records, which could endanger
their patients. It may be possible to reduce shift-related fatigue by using bright lights
in the workplace, minimizing shift changes, and taking scheduled naps.
Many people with total blindness experience life-long
sleeping problems because their retinas are unable to detect light. These people have a
kind of permanent jet lag and periodic insomnia because their circadian rhythms follow
their innate cycle rather than a 24-hour one. Daily supplements of melatonin may improve
night-time sleep for such patients. However, since the high doses of melatonin found in
most supplements can build up in the body, long-term use of this substance may create new
problems. Because the potential side effects of melatonin supplements are still largely
unknown, most experts discourage melatonin use by the general public.
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Sleep and Disease
Sleep and sleep-related problems play a role in a
large number of human disorders and affect almost every field of medicine. For example,
problems like stroke and asthma attacks tend to occur more frequently during the night and
early morning, perhaps due to changes in hormones, heart rate, and other characteristics
associated with sleep. Sleep also affects some kinds of epilepsy in complex ways. REM
sleep seems to help prevent seizures that begin in one part of the brain from spreading to
other brain regions, while deep sleep may promote the spread of these seizures. Sleep
deprivation also triggers seizures in people with some types of epilepsy.
Neurons that control sleep interact closely with the
immune system. As anyone who has had the flu knows, infectious diseases tend to make us
feel sleepy. This probably happens because cytokines, chemicals our immune systems
produce while fighting an infection, are powerful sleep-inducing chemicals. Sleep may help
the body conserve energy and other resources that the immune system needs to mount an
Sleeping problems occur in almost all people with mental
disorders, including those with depression and schizophrenia. People with depression, for
example, often awaken in the early hours of the morning and find themselves unable to get
back to sleep. The amount of sleep a person gets also strongly influences the symptoms of
mental disorders. Sleep deprivation is an effective therapy for people with certain types
of depression, while it can actually cause depression in other people. Extreme sleep
deprivation can lead to a seemingly psychotic state of paranoia and hallucinations in
otherwise healthy people, and disrupted sleep can trigger episodes of mania (agitation and
hyperactivity) in people with manic depression.
Sleeping problems are common in many other disorders as
well, including Alzheimerís disease, stroke, cancer, and head injury. These sleeping
problems may arise from changes in the brain regions and neurotransmitters that control
sleep, or from the drugs used to control symptoms of other disorders. In patients who are
hospitalized or who receive round-the-clock care, treatment schedules or hospital routines
also may disrupt sleep. The old joke about a patient being awakened by a nurse so he could
take a sleeping pill contains a grain of truth. Once sleeping problems develop, they can
add to a personís impairment and cause confusion, frustration, or depression.
Patients who are unable to sleep also notice pain more and may increase their requests for
pain medication. Better management of sleeping problems in people who have other disorders
could improve these patientsí health and quality of life.
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At least 40 million Americans each year suffer from
chronic, long-term sleep disorders each year, and an additional 20 million experience
occasional sleeping problems. These disorders and the resulting sleep deprivation
interfere with work, driving, and social activities. They also account for an estimated
$16 billion in medical costs each year, while the indirect costs due to lost productivity
and other factors are probably much greater. Doctors have described more than 70 sleep
disorders, most of which can be managed effectively once they are correctly diagnosed. The
most common sleep disorders include insomnia, sleep apnea, restless legs syndrome, and
Almost everyone occasionally suffers from short-term
insomnia. This problem can result from stress, jet lag, diet, or many other factors.
Insomnia almost always affects job performance and well-being the next day. About 60
million Americans a year have insomnia frequently or for extended periods of time, which
leads to even more serious sleep deficits. Insomnia tends to increase with age and affects
about 40 percent of women and 30 percent of men. It is often the major disabling symptom
of an underlying medical disorder.
For short-term insomnia, doctors may prescribe sleeping
pills. Most sleeping pills stop working after several weeks of nightly use, however, and
long-term use can actually interfere with good sleep. Mild insomnia often can be prevented
or cured by practicing good sleep habits (see "Tips for a Good Nightís
Sleep"). For more serious cases of insomnia, researchers are experimenting with
light therapy and other ways to alter circadian cycles.
Sleep apnea is a disorder of interrupted breathing during
sleep. It usually occurs in association with fat buildup or loss of muscle tone with
aging. These changes allow the windpipe to collapse during breathing when muscles relax
during sleep (see figure 3). This
problem, called obstructive sleep apnea, is usually associated with loud snoring
(though not everyone who snores has this disorder). Sleep apnea also can occur if the
neurons that control breathing malfunction during sleep.
During an episode of obstructive apnea, the personís
effort to inhale air creates suction that collapses the windpipe. This blocks the air flow
for 10 seconds to a minute while the sleeping person struggles to breathe. When the
personís blood oxygen level falls, the brain responds by awakening the person enough
to tighten the upper airway muscles and open the windpipe. The person may snort or gasp,
then resume snoring. This cycle may be repeated hundreds of times a night. The frequent
awakenings that sleep apnea patients experience leave them continually sleepy and may lead
to personality changes such as irritability or depression. Sleep apnea also deprives the
person of oxygen, which can lead to morning headaches, a loss of interest in sex, or a
decline in mental functioning. It also is linked to high blood pressure, irregular
heartbeats, and an increased risk of heart attacks and stroke. Patients with severe,
untreated sleep apnea are two to three times more likely to have automobile accidents than
the general population. In some high-risk individuals, sleep apnea may even lead to sudden
death from respiratory arrest during sleep.
An estimated 18 million Americans have sleep apnea.
However, few of them have had the problem diagnosed. Patients with the typical features of
sleep apnea, such as loud snoring, obesity, and excessive daytime sleepiness, should be
referred to a specialized sleep center that can perform a test called polysomnography.
This test records the patientís brain waves, heartbeat, and breathing during an
entire night. If sleep apnea is diagnosed, several treatments are available. Mild sleep
apnea frequently can be overcome through weight loss or by preventing the person from
sleeping on his or her back. Other people may need special devices or surgery to correct
the obstruction. People with sleep apnea should never take sedatives or sleeping pills,
which can prevent them from awakening enough to breathe.
Restless Legs Syndrome
Restless legs syndrome (RLS), a familial disorder
causing unpleasant crawling, prickling, or tingling sensations in the legs and feet and an
urge to move them for relief, is emerging as one of the most common sleep disorders,
especially among older people. This disorder, which affects as many as 12 million
Americans, leads to constant leg movement during the day and insomnia at night. Severe RLS
is most common in elderly people, though symptoms may develop at any age. In some cases,
it may be linked to other conditions such as anemia, pregnancy, or diabetes.
Many RLS patients also have a disorder known as periodic
limb movement disorder or PLMD, which causes repetitive jerking movements of
the limbs, especially the legs. These movements occur every 20 to 40 seconds and cause
repeated awakening and severely fragmented sleep. In one study, RLS and PLMD accounted for
a third of the insomnia seen in patients older than age 60.
RLS and PLMD often can be relieved by drugs that affect
the neurotransmitter dopamine, suggesting that dopamine abnormalities underlie these
disordersí symptoms. Learning how these disorders occur may lead to better therapies
in the future.
Narcolepsy affects an estimated 250,000 Americans. People
with narcolepsy have frequent "sleep attacks" at various times of the day, even
if they have had a normal amount of night-time sleep. These attacks last from several
seconds to more than 30 minutes. People with narcolepsy also may experience cataplexy
(loss of muscle control during emotional situations), hallucinations, temporary paralysis
when they awaken, and disrupted night-time sleep. These symptoms seem to be features of
REM sleep that appear during waking, which suggests that narcolepsy is a disorder of sleep
regulation. The symptoms of narcolepsy typically appear during adolescence, though it
often takes years to obtain a correct diagnosis. The disorder (or at least a
predisposition to it) is usually hereditary, but it occasionally is linked to brain damage
from a head injury or neurological disease.
Once narcolepsy is diagnosed, stimulants, antidepressants,
or other drugs can help control the symptoms and prevent the embarrassing and dangerous
effects of falling asleep at improper times. Naps at certain times of the day also may
reduce the excessive daytime sleepiness.
In 1999, a research team working with canine models identified a gene that causes narcolepsyóa breakthrough that brings a cure for this disabling condition within reach. The gene, hypocretin receptor 2, codes for a protein that allows brain cells to receive instructions from other cells. The defective versions of the gene encode proteins that cannot recognize these messages, perhaps cutting the cells off from messages that promote wakefulness. The researchers know that the same gene exists in humans, and they are currently searching for defective versions in people with narcolepsy.
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Sleep research is expanding and attracting more and more
attention from scientists. Researchers now know that sleep is an active and dynamic state
that greatly influences our waking hours, and they realize that we must understand sleep
to fully understand the brain. Innovative techniques, such as brain imaging, can now help
researchers understand how different brain regions function during sleep and how different
activities and disorders affect sleep. Understanding the factors that affect sleep in
health and disease also may lead to revolutionary new therapies for sleep disorders and to
ways of overcoming jet lag and the problems associated with shift work. We can expect
these and many other benefits from research that will allow us to truly understand
sleepís impact on our lives.
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Tips for a
Good Nightís Sleep:1 Set a schedule:
Go to bed at a set time each night and get up at the
same time each morning. Disrupting this schedule may lead to insomnia. "Sleeping
in" on weekends also makes it harder to wake up early on Monday morning because it
re-sets your sleep cycles for a later awakening.
Avoid caffeine, nicotine, and alcohol:
Try to exercise 20 to 30 minutes a day. Daily exercise
often helps people sleep, although a workout soon before bedtime may interfere with sleep.
For maximum benefit, try to get your exercise about 5 to 6 hours before going to bed.
Relax before bed:
Avoid drinks that contain caffeine, which acts as a
stimulant and keeps people awake. Sources of caffeine include coffee, chocolate, soft
drinks, non-herbal teas, diet drugs, and some pain relievers. Smokers tend to sleep very
lightly and often wake up in the early morning due to nicotine withdrawal. Alcohol robs
people of deep sleep and REM sleep and keeps them in the lighter stages of sleep.
Sleep until sunlight:
A warm bath, reading, or another relaxing routine can
make it easier to fall sleep. You can train yourself to associate certain restful
activities with sleep and make them part of your bedtime ritual.
Donít lie in bed awake:
If possible, wake up with the sun, or use very bright
lights in the morning. Sunlight helps the bodyís internal biological clock reset
itself each day. Sleep experts recommend exposure to an hour of morning sunlight for
people having problems falling asleep.
Control your room temperature:
If you canít get to sleep, donít just lie in
bed. Do something else, like reading, watching television, or listening to music, until
you feel tired. The anxiety of being unable to fall asleep can actually contribute to
See a doctor if your sleeping problem continues:
Maintain a comfortable temperature in the bedroom. Extreme
temperatures may disrupt sleep or prevent you from falling asleep.
If you have trouble falling asleep night after night,
or if you always feel tired the next day, then you may have a sleep disorder and should
see a physician. Your primary care physician may be able to help you; if not, you can
probably find a sleep specialist at a major hospital near you. Most sleep disorders can be
treated effectively, so you can finally get that good nightís sleep you need.
1Adapted from "When You
Canít Sleep: The ABCs of ZZZs," by the National Sleep Foundation.
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