How Sleep Works

Sleep is a period of rest that alternates with wakefulness. You have internal body clocks that control when you are awake and when your body is ready for sleep. These clocks have cycles of approximately 24 hours. The clocks are regulated by multiple factors, including light, darkness, and sleep schedules. Once asleep, you cycle through the stages of sleep throughout the night in a predictable pattern.

Sleep is important because it affects many of your body’s systems. Not getting enough sleep or enough quality sleep raises your risk for heart and respiratory problems and affects your metabolism and ability to think clearly and focus on tasks.

Explore this Health Topic to learn more about how sleep works, our role in research and clinical trials to improve health, and where to find more information.

Your Sleep/Wake Cycle - How Sleep Works

Many factors play a role in preparing your body to fall asleep and wake up. Your body has several internal clocks, called circadian clocks. These typically follow a 24-hour repeating rhythm, called the circadian rhythm. This rhythm affects every cell, tissue, and organ in your body and how they work. Learn more in our Circadian Rhythms Disorders Health Topic.

Your central circadian clock, located in your brain, tells you when it is time for sleep. Other circadian clocks are located in organs throughout your body. Your body’s internal clocks are in sync with certain cues in the environment. Light, darkness, and other cues help determine when you feel awake and when you feel drowsy. Artificial light and caffeine can disrupt this process by giving your body false wakefulness cues.

Urge to sleep
- How Sleep Works

Your body has a biological need for sleep that increases when you have been awake for a long time. This is controlled by homeostasis, the process by which your body keeps your systems, such as your internal body temperature, steady. A compound called adenosine is linked to this need for sleep. While you are awake, the level of adenosine in your brain continues to rise. The rising levels signal a shift toward sleep. Caffeine and certain drugs can interrupt this process by blocking adenosine.

Light-dark cycle
- How Sleep Works

If you follow a natural schedule of days and nights, light signals received through your eyes tell your brain that it is daytime. The area of your brain that receives these signals, called the suprachiasmatic nucleus, transmits the signals to the rest of your body through the sympathetic system and the parasympathetic system. This helps your central body clock stay in tune with the day and night. Exposure to artificial light interferes with this process.

The light–dark cycle influences when your brain makes and releases a hormones called melatonin. Melatonin travels to the cells in your body through your bloodstream. The amount of melatonin in your bloodstream starts to increase in the evening and peaks in the early morning. Melatonin is thought to promote sleep. As you are exposed to more light, such as the sun rising, your body releases another hormone called cortisol. Cortisol naturally prepares your body to wake up.

Exposure to bright artificial light in the late evening can disrupt this process and prevent your brain from releasing melatonin. This can make it harder to fall asleep. Examples of bright artificial light include the light from a TV screen, a smartphone, or a very bright alarm clock. Some people use physical filters or software to filter out some of the blue light from these devices.

Problems with your sleep/wake cycle and circadian clock
- How Sleep Works

Some people have problems with their sleep/wake cycle, meaning that their brain does not keep them awake or asleep at appropriate times. Examples include:

  • Insomnia. People who have insomnia have trouble falling asleep, staying asleep, or both. As a result, they may get too little sleep or not enough quality sleep. They may not feel refreshed when they wake up.
  • Narcolepsy. Narcolepsy causes periods of extreme daytime sleepiness. The disorder may also cause muscle weakness.

Sometimes, your central circadian clock is not properly aligned with your sleep time. Examples include:

  • Jet lag. Many people have trouble adjusting their sleep to fit a new time zone. This usually resolves within a few days.
  • Shift work disorder. People who work at night may have trouble sleeping during the day.

Did you know that your circadian clocks may be different from someone else’s and changes throughout your life?

Sleep Phases and Stages - How Sleep Works

When you sleep, you cycle through two phases of sleep: rapid eye movement (REM) and non-REM sleep. The cycle starts over every 80 to 100 minutes. Usually there are four to six cycles per night. You may wake up briefly between cycles. Sleep studies use sensors to record eye movements and brain activity, which are used to classify sleep phases and stages.

Non-REM sleep
- How Sleep Works

Non-REM sleep has three stages, defined by measurements of brain activity taken in sleep studies.

  • Stage 1. This stage is the transition between wakefulness and sleep.
  • Stage 2. When you reach stage 2, you are asleep.
  • Stage 3. This stage is called deep sleep or slow-wave sleep, after a particular pattern that appears in measurements of brain activity. You usually spend more time in this stage early in the night.

Rapid eye movement (REM) sleep
- How Sleep Works

During REM sleep, your eyes twitch and your brain is active. Brain activity measured during REM sleep is similar to your brain’s activity during waking hours. Dreaming usually happens during REM sleep. Your muscles normally become limp to prevent you from acting out your dreams. You usually have more REM sleep later in the night, but you do not have as much REM sleep in colder temperatures. This is because, during REM sleep, your body does not regulate its temperature properly.

How do our patterns of sleep change as we age?

Why Sleep Is Important - How Sleep Works

Sleep plays a vital role in good health and well-being throughout your life. The way you feel while you are awake depends in part on what happens while you are sleeping. During sleep, your body is working to support healthy brain function and maintain your physical health. In children and teens, sleep also helps support growth and development. Getting inadequate sleep over time can raise your risk for chronic health problems. It can also affect how well you think, react, work, learn, and get along with others. Learn how sleep affects your heart and circulatory system, metabolism, respiratory system, and immune system and how much sleep is enough.

Heart and circulatory system
- How Sleep Works

When you fall asleep and enter non-REM sleep, your blood pressure and heart rate fall. During sleep, your parasympathetic system controls your body, and your heart does not work as hard as it does when you are awake. During REM sleep and when waking, your sympathetic system is activated, increasing your heart rate and blood pressure to the usual levels when you are awake and relaxed. A sharp increase in blood pressure and heart rate upon waking has been linked to angina and heart attacks.

People who do not sleep enough or wake up frequently may have a higher risk of:

- How Sleep Works

Your body makes different hormones at different times of day. This may be related to your sleep pattern or your circadian clocks. In the morning, your body releases hormones that promote alertness, such as cortisol, which helps you wake up. Other hormones have 24-hour patterns that vary throughout your life; for example, in children, the hormones that tell the glands to release testosterone, estrogen, and progesterone are made in pulses at night, and the pulses get bigger as puberty approaches.

- How Sleep Works

The way your body handles fat varies according to various circadian clocks, including those in the liver, fat, and muscle. For example, the circadian clocks make sure that your liver is prepared to help digest fats at appropriate times. Your body may handle fat differently if you eat at unusual times.

Studies have shown that not getting enough quality sleep can lead to:

  • Increased levels of hormones that control hunger, including leptin and ghrelin, inside your body
  • Decreased ability to respond to insulin
  • Increased consumption of food, especially fatty, sweet, and salty foods
  • Decreased physical activity
  • Metabolic syndrome

All of these contribute to overweight and obesity.

Respiratory and immune systems
- How Sleep Works

During sleep, you breathe less often and less deeply and take in less oxygen. These changes can cause problems in people who have health problems such as asthma or chronic obstructive pulmonary disease (COPD). Asthma symptoms are usually worse during early morning sleep. Likewise, breathing problems in people who have lung diseases such as COPD can become worse during sleep.

Sleep also affects different parts of your immune system, which become more active at different times of day. For example, when you sleep, a particular type of immune cell works harder. That is why people who do not sleep enough may be more likely to get colds and other infections.

Problems with thinking and memory
- How Sleep Works

Sleep helps with learning and the formation of long-term memories. Not getting enough sleep or enough high-quality sleep can lead to problems focusing on tasks and thinking clearly. Read our Sleep Deprivation and Deficiency Health Topic for more information on how lack of sleep affects performance of daily activities, including driving and schoolwork.

How much sleep is enough
- How Sleep Works

Experts recommend that adults sleep between 7 and 9 hours a night. Adults who sleep less than 7 hours a night may have more health issues than those who sleep 7 or more hours a night. Sleeping more than 9 hours a night is not necessarily harmful and may be helpful for young adults, people who are recovering from sleep deprivation, and people who are sick.

How much sleep children should get depends on their age. Sleep experts consider naps to be appropriate for children under age 7. Below you can find the recommended hours of sleep, including naps, for different ages.

  • For newborns younger than 4 months, sleep patterns vary widely.
  • Babies 4 months to 1 year old should sleep 12 to 16 hours per day.
  • Children 1 to 2 years old should sleep 11 to 14 hours per day.
  • Children 3 to 5 years old should sleep 10 to 13 hours per day.
  • Children 6 to 12 years old should sleep 9 to 12 hours per day.
  • Teens 13 to 18 years old should sleep 8 to 10 hours per day.

Talk to your doctor or your child’s doctor if you think you or your child is sleeping too much or too little.

Research for Your Health

The NHLBI is part of the U.S. Department of Health and Human Services’ National Institutes of Health (NIH)—the Nation’s biomedical research agency that makes important scientific discoveries to improve health and save lives. We are committed to advancing science and translating discoveries into clinical practice to promote the prevention and treatment of sleep disorders. Learn about current and future NHLBI efforts to improve health through research and scientific discovery.

Improving health with current research
- How Sleep Works

Learn about the following ways the NHLBI continues to translate current research into improved understanding of sleep and sleep disorders. Research on this topic is part of the NHLBI’s broader commitment to advancing scientific discovery for sleep science and sleep disorders.

  • NHLBI’s National Center on Sleep Disorders Research (NCSDR). The NCSDR administers sleep research projects, training, and educational awareness programs, and serves as an NIH point-of-contact for federal agencies and public interest organizations. NCSDR also participates in research translation, as well as the dissemination of scientific sleep and circadian advances to healthcare professionals, public health officials, and the public.
  • Improving the Quality of Medical School Education on Sleep Disorders. As part of its efforts to ensure that research advances are utilized by healthcare providers, the NCSDR has supported the development of medical school sleep disorder curricula and durable educational materials.
  • Research Conference on Sleep and the Health of Women. This 2018 conference focused on the importance of sleep for women’s health. It showcased a decade of federally funded research advances in understanding both the health risks and costs to society as well as treatment options associated with sleep deficiency and sleep disorders in women. Learn more from the 2018 Research Conference on Sleep and the Health of Women.
  • Sleep Disorders Research Advisory Board (SDRAB). The NHLBI has administered this specialty program advisory panel since 1993. Board members, including medical professionals, federal partners, and members of the public, meet regularly to provide feedback to the NIH on sleep-related research needs and to discuss how to move sleep research forward. Visit the Sleep Disorders Research Advisory Board for more information.
  • National Sleep Research Resource (NSRR). This resource was established by the NHLBI to provide biomedical researchers a large, well-characterized data collection from NIH-funded sleep research studies. These data can be used in new research studies to advance sleep research.

Learn more about how the NHLBI is contributing to knowledge about sleep.

Advancing research for improved health
- How Sleep Works

In support of our mission, we are committed to advancing sleep research, in part through the following ways.

Learn more about the exciting sleep-related research areas we are exploring.

Participate in NHLBI Clinical Trials

We lead or sponsor many studies on sleep. See if you or someone you know is eligible to participate in our clinical trials.

Are you an adult with a diagnosed sleep phase disorder?

This study aims to assess the sleep patterns and quality of sleep in people who have sleep phase disorders to determine how the disorders affect their circadian rhythms. To participate in this study, you must be at least 18 years old and have a sleep phase disorder, such as advanced sleep-wake phase syndrome or delayed sleep-wake phase syndrome. This study is located in Chicago, Illinois.

Is your child black and 8 to 11 years old?

This study is testing a way of enhancing sleep in children with the goal of preventing obesity. To participate in this study, your child must be 8 to 11 years old and must be black or African American. This study is located in Philadelphia, Pennsylvania.

Do you have overweight?

Circadian rhythm disorders can cause overweight and obesity. This study is examining how not getting enough sleep causes these complications. To participate in this study, you must be between the ages of 20 and 40 and have overweight but not obesity. This study is located in New York, New York.

Are you an adult who does not work nights?

This study is examining how a person’s sleep schedule affects circadian rhythms, learning skills, physical ability, and mood. To participate in this study, you must be between 18 and 35 years old and not have underweight or obesity. This study is located in Boston, Massachusetts.

Are you pregnant, with a BMI of 30 or higher?

This study is examining the connection between obesity, breathing problems associated with sleep, and pregnancy problems such as preeclampsia. To participate in this study, you must be 18 or older, pregnant, and have obesity. This study is located in Pittsburgh, Pennsylvania.

Are you black, 18 to 35 years old, and interested in joining a study about how sleep affects blood vessels?

This study is testing how your sleep affects your blood vessels, and whether personalized recommendations can improve your sleep. To participate in this study, you must be 18 to 35 years old, and identify as black or African American. This study is located in Washington, D.C.
View more information about Stress, Sleep and Cardiovascular Risk.

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