Brain Basics: Understanding Sleep
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.
z Sleep: A Dynamic Activity
z How Much Sleep Do We Need?
z What Does Sleep Do For Us?
z Dreaming and REM Sleep
z Sleep and Circadian Rhythms
z Sleep and Disease
z Sleep Disorders
z The Future
z Tips for a Good Night's Sleep
Sleep: A Dynamic Activity
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 (seefigure 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 REM sleep.
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 deep sleep.
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 — 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 they’ve 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
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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 — 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 don’t 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 Need?
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 don’t 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 haven’t 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 alcohol’s 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 brain’s last step before falling asleep, driving while
drowsy can – and often does – 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 can’t stop yawning, or if
you can’t remember driving the last few miles, you are probably too drowsy to drive safely.
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What Does Sleep Do For Us?
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 rats’ immune systems become impaired. Some studies
suggest that sleep deprivation affects the immune system in detrimental ways.
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
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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 body’s 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 improve learning.
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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 (seefigure 2). These signals travel to a brain
region called the thalamus, which relays them to the cerebral cortex — 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 — 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 the dream.
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 cortex’s 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 Rhythms
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 body’s biological "clock." This clock, called the suprachiasmatic
nucleus or SCN (seefigure 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 body’s 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 people’s 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
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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 "time givers").
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 body’s clock. You will feel
tired when the alarm rings at 8 a.m. the next morning because, according to your body’s 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 people’s 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 attack.
Sleeping problems occur in almost all people with mental disorders, including those with depress