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.
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.
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.
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:
Sometimes, your central circadian clock is not properly aligned with your sleep time. Examples include:
Did you know that your circadian clocks may be different from someone else’s and changes throughout your life?
Circadian clocks are different for different people. Most people’s natural circadian cycle is slightly greater than 24 hours. Some people naturally wake up early and some naturally stay up late. For example, it is natural for many teens to prefer later bedtimes and to sleep later in the morning than adults.
The rhythm and timing of the body clocks also decline with age. Neurons, or cells, in the brain that promote sleep are lost as part of normal aging. Certain conditions such as Alzheimer’s disease can also speed the loss of neurons. This makes it harder for older adults to stay asleep. Other factors, such as less physical activity or less time spent outdoors, also affect circadian rhythms. As a result, older adults usually sleep less and wake up earlier.
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 has three stages, defined by measurements of brain activity taken in sleep studies.
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?
The patterns and types of sleep change as people mature. For example, newborns spend more time in REM sleep. The amount of slow-wave sleep peaks in early childhood and then drops sharply in the teenage years. Slow-wave sleep continues to decrease through adulthood, and older people may not have any slow-wave sleep at all.
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.
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:
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.
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:
All of these contribute to overweight and obesity.
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.
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.
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.
Talk to your doctor or your child’s doctor if you think you or your child is sleeping too much or too little.
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.
Learn more about how the NHLBI is contributing to knowledge about sleep.
Learn about some of the pioneering research contributions we have made over the years that have improved clinical care.
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.
We lead or sponsor many studies on sleep. See if you or someone you know is eligible to participate in our clinical trials.
After reading our How Sleep Works Health Topic, you may be interested in additional information found in the following resources.