Sleep is a complex biological process that helps people process new information, stay healthy, and re-energize. Periods of sleep and wakefulness are part of how our bodies function.

Although you are resting while you sleep, your brain remains highly active. Sleep consists of different stages that repeat several times each night. During sleep, the brain cycles through two distinct phases: rapid eye movement (REM) sleep and non-REM sleep. Not completing the full sleep process can stress your body.

Why is sleep important?

Each sleep phase and stage is important to ensure that the mind and body are completely rested. Certain stages help you feel rested and energetic the next day, while other stages help you learn information and form memories. Sleep is important in the function of your body’s other systems, such as your metabolism and immune system. Sleep may also help your body clear toxins from your brain that build up while you were awake.

Not getting enough or enough quality sleep contributes, in the short term, to problems with learning and processing information, and it can have a harmful effect on long-term health and well-being. According to the Centers for Disease Control and Prevention (CDC), many U.S. adults report that they don’t get the recommended number of hours of sleep each night.

Sleep affects how well you do your daily tasks, your mood, and your health in the following ways:

• Performance. Cutting back on sleep by as little as 1 hour can make it difficult to focus the next day and can slow your response time. Insufficient sleep can also make you more likely to take risks and make poor decisions, according to the National Heart, Lung, and Blood Institute (NHLBI).
• Mood. Sleep affects your mood. Insufficient sleep can make you more easily annoyed or angry, and that can lead to trouble with relationships, particularly for children and teens. Also, people who don’t get enough sleep are more likely to become depressed, according to NHLBI.
• Health. Sleep is important for good health. Research in adults has shown that lack of sleep or lack of quality sleep increases a person’s risk for high blood pressure, heart disease, and other medical conditions. Your environment can affect the quality of your sleep by causing disturbances that prevent you from sleeping through the night. Also, during sleep the body produces hormones that help the body grow and, throughout life, build muscle, fight illnesses, and repair damage to the body. Growth hormone, for example, is produced during sleep, and it is essential for growth and development. Other hormones produced during sleep affect how the body uses energy, which may explain why lack of sleep contributes to obesity and diabetes.

Sleep is a complex biological process. While you are sleeping, you are unconscious, but your brain and body functions are still active. They are doing a number of important jobs that help you stay healthy and function at your best. So when you don't get enough quality sleep, it does more than just make you feel tired. It can affect your physical and mental health, thinking, and daily functioning.

Sleep and wakefulness are generally regulated by our brains working with input from our senses and our circadian (pronounced sur-KAY-dee-uhn) clock.

This system pushes us to wake up and remain awake at certain times and pushes us to sleep at certain times. Research has helped us to begin to understand this system at the level of the cells in the brain. More work is needed to understand exactly how the brain, senses, and our body’s clock work in waking and sleeping and what can happen to disturb this cycle. Visit the National Institute of Neurological Disorders and Stroke’s Understanding Sleep page for more information on the sleep cycle.

Sleep Drive

The need for sleep is driven by the length of time you are awake. The longer you are awake, the greater your “drive” or need to sleep. The drive to sleep continues to build within your body until you are able to sleep.

Circadian Clock

Your body has a natural clock, called a “circadian clock,” that helps you regulate your sleep. The word “circadian” refers to rhythmic biological cycles that repeat about every 24 hours. These cycles are also called circadian rhythms. Your circadian clock is strongly influenced by light, which is the reason why people living in different regions have different sleeping schedules. This is also the reason why people who work night shifts can have difficulty falling asleep or staying awake.

At bedtime, when your drive to sleep is greatest, your sleep drive and circadian clock work together to allow you to fall asleep. After you have slept for some time, when your drive to sleep is lower, your circadian clock allows you to stay asleep until the end of the night.

Circadian Rhythms

Circadian rhythms regulate changes in the brain and body that occur over the course of a day. Your body’s biological clock controls most circadian rhythms. This clock is found in a region of the brain called the hypothalamus (pronounced hy-puh-THAL-uh-muhs). The hypothalamus affects sleep and arousal.

Light detected by special neurons in the eye sends signals to many areas of the brain, including the hypothalamus. Signals from the hypothalamus travel to different regions of the brain, including the pineal (pronounced PIN-ee-uhl) gland. In response to light, such as sunlight, the pineal gland turns off the production of melatonin, a hormone that causes a feeling of drowsiness. The levels of melatonin in the body normally increase after darkness, which makes you feel drowsy.

The change in melatonin during the sleep/wake cycle reflects circadian rhythms. During sleep, the hypothalamus also controls changes in body temperature and blood pressure.

Because circadian rhythms are controlled by light, people who have some degree of blindness in both eyes may have trouble sleeping. For more information about how vision affects circadian rhythms, visit the National Institute of Neurological Disorders and Stroke page on understanding sleep.

In broad terms, the brain of someone who is sleeping cycles through two basic phases: rapid eye movement (REM) sleep and non-REM sleep. Non-REM sleep includes three different stages. A person cycles through REM sleep and non-REM sleep several times a night.

For more detailed information about what happens during sleep and phases of sleep, check out Your Guide to Healthy Sleep, from the National, Heart, Lung, and Blood Institute.

Each phase of sleep helps the mind and body stay rested. Certain stages help you feel rested and energetic the next day, while both phases help you learn information and form memories.

Sleep progresses in a cycle: from non-REM sleep stage 1 to non-REM sleep stage 2, to non-REM sleep stage 3, to REM sleep. Then the process starts over again with non-REM sleep stage 1.

The length of sleep stages changes during a given night’s sleep. For example, near the beginning of sleep, the body cycles through relatively short periods of REM sleep and long periods of deep sleep. As the night goes on, periods of REM sleep increase and those of deep sleep decrease. Near the end of a night of sleep, a person spends nearly all of their time in stages 1 and 2 and REM.

Some of the characteristics of each phase include the following.

Non-REM Sleep

As you begin to fall asleep, you enter non-REM sleep, which consists of stages 1 through 3, as follows:

• Stage 1
  • You are in between being awake and being asleep.
  • Your heartbeat and breathing slow and your muscles relax.
• Stage 2
  • You are in a light sleep.
  • Your brain waves slow down.
  • Your body temperature lowers.
• Stage 3
  • Your deepest and most restorative sleep happens.
  • Your heartbeat and breathing slow to their lowest levels.
  • Your muscles relax.
  • Your body increases the supply of blood to your muscles.
  • Your body performs tissue growth and repair.
  • Your energy is restored.
  • Your body releases hormones.

REM Sleep

You first enter REM sleep about 90 minutes after you fall asleep. REM sleep becomes longer later into the night. REM is characterized as follows:

• Your brain and body are energized.
• Your breathing becomes fast and irregular.
• Your brain is active and dreaming occurs.
• Your eyes dart back and forth.
• Your body becomes immobile and relaxed, preventing you from acting out your dreams.
• Your body temperature is not as tightly regulated.

REM sleep begins in response to signals sent to and from different regions of the brain. Signals are sent to the brain’s cerebral cortex, which is responsible for learning, thinking, and organizing information. Signals are also sent to the spinal cord to shut off movement, creating a temporary inability to move the muscles (“paralysis”) in the arms and legs. If this temporary paralysis is disrupted, people might move while they are dreaming (“sleepwalking”). A person who sleepwalks is at risk for injury.

REM sleep is thought to be involved in storing memories, learning, and balancing mood. REM sleep stimulates regions of the brain that are used for learning. Studies have shown that when people are deprived of REM sleep, they are not able to remember what they were taught before going to sleep. Lack of REM sleep has also been linked to certain health conditions, such as migraines. However, insufficient sleep, regardless of sleep stage, can interfere with learning, memory, and performance. If you have any concerns about your sleep quality and habits, speak with your healthcare provider.

Dreaming

Scientists aren’t sure why we dream. While some of the signals sent to the cortex during sleep are important for learning and memory, some signals seem to be random. Dreams are generally most vivid during REM sleep, but dreaming can also occur during non-REM sleep.

Through research, we are learning more about dreaming. One study, for example, found that a pattern of brain activity from a part of the cortex near the back of the brain is a good predictor of whether an individual is dreaming, whether the individual was in the REM or non-REM sleep.

Sleep disorders are conditions that disturb your normal sleep patterns. There are more than 80 different sleep disorders. Some major types include:

• Insomnia - being unable to fall asleep and stay asleep. This is the most common sleep disorder.
• Sleep apnea - a breathing disorder in which you stop breathing for 10 seconds or more during sleep
• Restless leg syndrome (RLS) - a tingling or prickly sensation in your legs, along with a powerful urge to move them
• Hypersomnia - being unable to stay awake during the day. This includes narcolepsy, which causes extreme daytime sleepiness.
• Circadian rhythm disorders - problems with the sleep-wake cycle. They make you unable to sleep and wake at the right times.
• Parasomnia - acting in unusual ways while falling asleep, sleeping, or waking from sleep, such as walking, talking, or eating

Some people who feel tired during the day have a true sleep disorder. But for others, the real problem is not allowing enough time for sleep. It's important to get enough sleep every night. The amount of sleep you need depends on several factors, including your age, lifestyle, health, and whether you have been getting enough sleep recently. Most adults need about 7-8 hours each night.

According to the National Institute of Neurological Disorders and Stroke, the amount of sleep people need depends on several factors, including their age, individual requirements, and whether they have been getting adequate sleep.

The National Sleep Foundation recommends how much sleep people should get at each age. The recommendations are based on a review of more than 300 research studies.

Although the amount of sleep you get each day is important, other aspects of your sleep also contribute to your health and well-being. Good sleep quality is also essential. Signs of poor sleep quality include not feeling rested even after getting enough sleep, repeatedly waking up during the night, and experiencing symptoms of sleep disorders (such as snoring or gasping for air). Improving sleep quality may be helped by better sleep habits or being diagnosed and treated for any sleep disorder you may have.

Sleep experts recommend 7 to 9 hours of sleep for most adults. While sleep patterns may change as we age, the amount of sleep the body needs does not usually change. Older people may wake up more frequently throughout the night and end up getting less sleep during the overnight hours. Older adults are more likely to be taking medicines that interfere with sleep. However, their need for sleep is not drastically less than that of younger adults. Older people may take more naps during the day because they get less sleep at night.

Many people report that they experience problems with sleep. But what does that really mean?

Sleep problems differ in how severe they are and in their specific features. For example, how long you sleep or are awake, how deeply you sleep, and how you fall asleep are all related to certain sleep problems.

In the past, healthcare providers focused on sleep deprivation—not getting enough sleep (also called insufficient sleep)—as a main type of sleep problem. But research has shown that sleep deficiency, a concept that describes a group of sleep problems, is actually a more accurate way to talk about sleep problems.

Sleep deficiency is a term that describes several different sleep issues, including:

• Not getting enough sleep (sleep deprivation or insufficient sleep)
• Sleeping at the wrong time of day (not in sync with your body’s natural clock)
• Not sleeping well or not spending enough time in certain stages of sleep
• Having a sleep disorder

Sleep deficiency is associated with a number of health and other problems ranging from feeling tired to chronic health conditions.

Sleep disorders are a group of more than 70 major sleep problems with symptoms that can include inappropriate sleep, severe sleep deprivation, and periods of stopped breathing. People with sleep disorders usually benefit from treatments and monitoring from a healthcare provider.

Sleep-related difficulties affect many people. The following is a description of some of the major sleep disorders. If you, or someone you know, is experiencing any of the following, it is important to receive an evaluation by a healthcare provider or, if necessary, a provider specializing in sleep medicine.

Insomnia

Insomnia is characterized by an inability to initiate or maintain sleep. It may also take the form of early morning awakening in which the individual awakens several hours early and is unable to resume sleeping. Difficulty initiating or maintaining sleep may often manifest itself as excessive daytime sleepiness, which characteristically results in functional impairment throughout the day. Before arriving at a diagnosis of primary insomnia, the healthcare provider will rule out other potential causes, such as other sleep disorders, side effects of medications, substance abuse, depression, or other previously undetected illness. Chronic psychophysiological insomnia (or “learned” or “conditioned” insomnia) may result from a stressor combined with fear of being unable to sleep. Individuals with this condition may sleep better when not in their own beds. Health care providers may treat chronic insomnia with a combination of use of sedative-hypnotic or sedating antidepressant medications, along with behavioral techniques to promote regular sleep.

Narcolepsy

Excessive daytime sleepiness (including episodes of irresistible sleepiness) combined with sudden muscle weakness are the hallmark signs of narcolepsy. The sudden muscle weakness seen in narcolepsy may be elicited by strong emotion or surprise. Episodes of narcolepsy have been described as “sleep attacks” and may occur in unusual circumstances, such as walking and other forms of physical activity. The healthcare provider may treat narcolepsy with stimulant medications combined with behavioral interventions, such as regularly scheduled naps, to minimize the potential disruptiveness of narcolepsy on the individual’s life.

Restless Legs Syndrome (RLS)

RLS is characterized by an unpleasant “creeping” sensation, often feeling like it is originating in the lower legs, but often associated with aches and pains throughout the legs. This often causes difficulty initiating sleep and is relieved by movement of the leg, such as walking or kicking. Abnormalities in the neurotransmitter dopamine have often been associated with RLS. Healthcare providers often combine a medication to help correct the underlying dopamine abnormality along with a medicine to promote sleep continuity in the treatment of RLS.

Sleep Apnea

Snoring may be more than just an annoying habit – it may be a sign of sleep apnea. Persons with sleep apnea characteristically make periodic gasping or “snorting” noises, during which their sleep is momentarily interrupted. Those with sleep apnea may also experience excessive daytime sleepiness, as their sleep is commonly interrupted and may not feel restorative. Treatment of sleep apnea is dependent on its cause. If other medical problems are present, such as congestive heart failure or nasal obstruction, sleep apnea may resolve with treatment of these conditions. Gentle air pressure administered during sleep (typically in the form of a nasal continuous positive airway pressure device) may also be effective in the treatment of sleep apnea. As interruption of regular breathing or obstruction of the airway during sleep can pose serious health complications, symptoms of sleep apnea should be taken seriously. Treatment should be sought from a health care provider.

Circadian rhythms are disrupted when people travel from one time zone to another. The feeling that you experience when your circadian rhythms (biological cycles) are disrupted is called “jet lag.” The reason for jet lag is the change in time zones. For example, traveling from California to New York makes your body’s biological clock “lose” 3 hours. When you are in New York and your alarm rings at 8:00 a.m., you will feel tired and groggy because your body is still on California time, which would be 5:00 a.m. It will take your body a few days to adjust to the new time zone, but the adjustment will eventually take place. After a couple of days, you will find that 8:00 a.m. feels like the correct time to wake up if that is part of your normal schedule and you have had adequate sleep.

Some studies have shown that supplements of melatonin, a hormone that is produced by the body and sold as a treatment for insomnia, can help treat jet lag. This supplement has been especially effective for people crossing five or more time zones and for those traveling east. However, additional studies are needed to test the safety and effectiveness of melatonin for insomnia and jet lag; few studies are available, and it has not been tested for long-term use. Before you take any kind of supplement, be sure to check with your healthcare provider.

SIDS is the sudden death of an infant younger than 1 year old that remains unexplained even after a thorough investigation. SIDS is viewed as a “sleep-related” cause of infant death because infants die during sleep. It is a leading cause of death in children between 1 month and 1 year of age.

NICHD leads the Safe to Sleep campaign (formerly the Back to Sleep campaign) to educate parents, caregivers, and healthcare providers about ways to reduce the risk of SIDS and other sleep-related causes of infant death, such as accidental suffocation.

Healthcare providers don’t know what exactly causes SIDS, so there is no sure way to prevent SIDS. However, there are ways to reduce baby’s risk of SIDS and other sleep-related causes of infant death, including:

• Always place infants on their backs to sleep, for naps and at night. Infants who sleep on their backs are less likely to die of SIDS than infants who sleep on their stomachs or sides. Placing your baby on his or her back to sleep is the number one way to reduce the risk of SIDS.
• Place baby on a firm, flat sleep surface, such as on a mattress in a safety-approved* crib covered by a fitted sheet with no other loose bedding or soft items in the sleep area.
• Breastfeed your baby to reduce the risk of SIDS.
• Share your room with baby. Keep baby in your room close to your bed, but on a separate surface designed for infants, ideally for baby’s first year but at least for the first 6 months.

There are different causes for different sleep disorders, including:

• Other conditions, such as heart disease, lung disease, nerve disorders, and pain
• Mental illnesses, including depression and anxiety
• Medicines
• Genetics

Sometimes the cause is unknown.

There are also some factors that can contribute to sleep problems, including:

• Caffeine and alcohol
• An irregular schedule, such as working the night shift
• Aging. As people age, they often get less sleep or spend less time in the deep, restful stage of sleep. They are also more easily awakened.

Medical conditions, including pregnancy, some intellectual and developmental disabilities (IDDs), depression, Alzheimer’s disease, and cancer, may disrupt natural or healthy sleep patterns.

• Pregnancy.Many women who become pregnant find that they develop sleeping problems during pregnancy that they did not have before they got pregnant. Fatigue is common during pregnancy, especially during the first and third trimesters. Pregnant women may develop problems such as insomnia, restless legs syndrome, sleep apnea, nighttime heartburn―which can cause long-term damage to the esophagus―and frequent nighttime urination. All of these problems can affect the quality of sleep as well as sleep duration.
• IDDs. IDDs, particularly among children, are associated with sleeping problems. For example, children with autism spectrum disorders or attention deficit/hyperactivity disorder often have sleep problems. The specific treatment or intervention depends on the IDD, the child’s age, and other factors.
• Other conditions.Sleeping problems are common in people with intellectual disabilities or mental health problems, such as depression or schizophrenia. Sleeping difficulties are also common in many other disorders and illnesses, including Alzheimer’s disease, stroke, cancer, and head injury. Some researchers believe that many of these sleep problems happen because of changes in the brain areas that control sleep and waking. Medicines used to control the symptoms of certain conditions (cancer, for example) can also cause sleep problems.

The symptoms of sleep disorders depend on the specific disorder. Some signs that you may have a sleep disorder include that:

• You regularly take more than 30 minutes each night to fall asleep
• You regularly wake up several times each night and then have trouble falling back to sleep, or you wake up too early in the morning
• You often feel sleepy during the day, take frequent naps, or fall asleep at the wrong times during the day
• Your bed partner says that when you sleep, you snore loudly, snort, gasp, make choking sounds, or stop breathing for short periods
• You have creeping, tingling, or crawling feelings in your legs or arms that are relieved by moving or massaging them, especially in the evening and when trying to fall asleep
• Your bed partner notices that your legs or arms jerk often during sleep
• You have vivid, dreamlike experiences while falling asleep or dozing
• You have episodes of sudden muscle weakness when you are angry or fearful, or when you laugh
• You feel as though you cannot move when you first wake up

The act of snoring, by itself, is often harmless. However, in some people, it can signal a life-threatening disorder called sleep apnea. Sleep apnea causes pauses in your breathing that can last for seconds or minutes, dozens of times each hour. These pauses disrupt your sleep and cause you to wake up or sleep only lightly, which can make you feel extremely tired during the day. The pauses also reduce the amount of oxygen in your blood and can cause damage to the heart and blood vessels, which increases the risk of heart disease. Sleep apnea also increases the risk for high blood pressure, stroke, and diabetes.

Fortunately, sleep apnea is treatable. People who snore loudly, have pauses in breathing during sleep, and feel very tired during the day should speak with a healthcare provider.

Inadequate sleep—such as not sleeping long enough overall or having poor quality sleep—affects both how we feel and how we function.

Short-term effects can include drowsiness or sleepiness, irritability, reduced alertness, poor motor skills, and attention problems.

Inadequate sleep over a longer period of time can affect how the body functions. For example:

• NIH research shows that not getting enough sleep is associated with excess body weight and obesity. All age groups, including children, seem to be affected in the same way. In addition, blood samples from people who get little sleep show metabolic changes that are similar to those seen in obese people.
• Sleep duration is associated with type 2 diabetes. A review of 10 studies involving more than 18,000 people found that people who got between 7 and 8 hours of sleep per day have the lowest risk of diabetes. Those who sleep more or less than that 7 to 8 hours have a higher risk of having diabetes.
• Sleep apnea, a condition in which a person temporarily stops breathing while sleeping, increases risk for several different cardiovascular conditions, including high blood pressure (hypertension), stroke, coronary heart disease, and irregular heartbeat.
• Research has also found that inadequate sleep increases blood pressure levels in people who already have hypertension. This finding may be one reason why people who lack sleep over time have an increased risk of cardiovascular disease.
• Inadequate sleep, especially over a long period of time, can lead or contribute to mood disorders. Chronic sleep problems have been linked to depression, anxiety, and mental distress. For example, one study reported that participants who slept only 4.5 hours per night were more stressed, sad, angry, and mentally exhausted than a comparison group with longer sleep. In extreme cases, sleep deprivation is also linked to psychosis, paranoia, and hallucinations. Disrupted sleep can also trigger manic episodes, including agitation and hyperactivity, in people with bipolar disorder, a mood disorder in which people may cycle between extreme highs and extreme lows in their mood.
• Sleep disturbance has been linked to pain and disability. For example, in a recent NIH-supported study of arthritis patients, sleep disturbance was associated with pain and depression and predicted disability. In another study of arthritis patients with knee or hip replacement, researchers found that pain was associated with disturbed sleep but also that disturbed sleep was linked specifically to pain symptoms.

To make a diagnosis, your health care provider will use your medical history, your sleep history, and a physical exam. You may also have a sleep study (polysomnogram). The most common types of sleep studies monitor and record data about your body during a full night of sleep. The data includes:

• Brain wave changes
• Eye movements
• Breathing rate
• Blood pressure
• Heart rate and electrical activity of the heart and other muscles

Other types of sleep studies may check how quickly you fall asleep during daytime naps or whether you are able to stay awake and alert during the day.