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.

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 nervous system and the parasympathetic nervous 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 hormone 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:

• 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?

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.

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?

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:

• High blood pressure
• Ischemic heart disease
• Obesity
• Stroke

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:

• 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.

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.

• 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.