Sleep

Sleep is a special time when our minds and bodies rest and get better. While we are asleep, our brain stays busy in different ways, and we often dream. Sleep helps our bodies heal and keeps our memories and thoughts working well. It is very important for staying healthy and feeling good during the day.

Our sleep happens in cycles, switching between two types: rapid eye movement (REM) sleep, when we usually dream, and non-REM sleep, when our body repairs itself. The brain cleans away waste during sleep, which is why rest is so important for learning and growing.

Many things can affect how we sleep, like using screens before bed. Light from devices can trick our brains into thinking it is still daytime, making it harder to fall asleep. Good sleep habits help us get the rest we need to feel our best.

Physiology

Main article: Neuroscience of sleep

When we sleep, our brain uses less energy than when we are awake, especially during deep sleep. This helps the brain get its energy back. Sleep also makes us less aware of sounds and other things around us, but we can still hear loud noises.

During deep sleep, our bodies release growth hormone, which helps us grow. Scientists study sleep using special machines that track brain waves, eye movements, and muscle activity. Sleep has two main types: non-REM sleep, when our body relaxes deeply, and REM sleep, when we dream and our brains are very active. Our sleep moves between these types about every 90 minutes.

Timing

Sleep timing is guided by the circadian clock, sleep-wake homeostasis, and by personal choice.

Sleep timing mostly depends on hormonal signals from the circadian clock, or Process C. This is a system that uses cues from the environment to create a day–night rhythm inside the body. Process C works against the need for sleep during the day (in diurnal animals) and supports it at night. The suprachiasmatic nucleus (SCN) in the brain is a key part of this process.

The longer we stay awake, the more we feel the need to sleep. This is called Process S. The balance between sleeping and waking is managed by a process called homeostasis. Not getting enough sleep is called sleep deprivation.

Humans are also affected by social time, like when others are awake, work hours, and the time shown on clocks. Time zones help organize time for people in the same area, but they don’t perfectly match the sun’s natural rising and setting. For example, China spans five time zones but officially uses only one.

In polyphasic sleep, a person sleeps several times in a day. In monophasic sleep, sleep happens all at once. In experiments, humans sometimes sleep more often if they have nothing else to do. Long ago, before the Industrial Revolution, people often slept in two periods.

Different sleep patterns, like being an "early bird" or a "night owl", are called chronotypes. Genetics, habits, and age can affect chronotype. Sleep patterns can change over a person’s life. Very unusual sleep patterns are called circadian rhythm sleep disorders.

Naps are short daytime sleeps that help people rest. Many children nap, and some adults do too. The best nap lasts about 10–20 minutes. Sleeping longer can make it hard to wake up and may leave a person feeling slow. This is called sleep inertia.

Ideal duration

Sleep needs change as we grow older, and everyone is a little different. Experts say that sleeping about 6–7 hours each night is good for health.

Children especially need lots of sleep. By age two, a child’s brain is almost fully grown, and sleep helps them learn and remember new things. Having a regular bedtime routine — like reading a story or a warm bath — can help children sleep better.

Children need many hours of sleep each day to grow and do well. For example, newborns need up to 18 hours of sleep, but this amount gets smaller as children get older. In 2015, the National Sleep Foundation in the US shared new recommendations for how much sleep children should get at different ages.

Main articles: Infant sleep and Adolescent sleep

Functions

Sleep helps keep our bodies and minds healthy. While we sleep, the brain clears out waste and repairs itself. This is helped by a system called the glymphatic system. Sleep is important for our bodies, too. It helps the brain process and store memories better, especially during slow-wave sleep and REM sleep.

When we sleep, especially during REM sleep, we often dream. Dreams can feel very real while we are sleeping, even if they seem strange when we wake up. Some people keep dream journals to remember their dreams better.

Disorders

Insomnia

Main article: Insomnia

See also: Psychological stress and sleep

Insomnia is when people have trouble falling asleep or staying asleep. It’s very common. Causes can include stress, a noisy or bright room, irregular sleep times, or too much excitement before bed. To help with insomnia, it’s useful to keep a regular sleep schedule and avoid stressful activities before bedtime. Making the bedroom a calm place without screens can also help.

Exercise often helps improve sleep, especially if done several hours before bedtime. Using white noise, like the sound of rain or fans, can also help people fall asleep better.

Sleep health

Sleep duration means how long you sleep. Sleep quality means how fast you fall asleep and if you stay asleep. Good sleep helps you feel better and manage your feelings well.

Not enough sleep or poor sleep can cause health problems. This can include heart disease, being overweight, and mental health issues. Both kids and adults can have trouble sleeping. Good sleep habits can help. These include going to bed and waking up at the same time each day, not using screens before bed, and having a comfortable place to sleep.

Sleep and musculoskeletal health

Sleeping positions and the type of pillows and mattresses you use can affect your back and neck. Sleeping on your back or side is usually better for your spine than sleeping on your stomach. If you sleep on your side, putting a pillow between your knees can help keep your legs straight. Using the right pillow height can help keep your neck and spine straight.

Mattresses that are medium-firm to firm are often good for helping prevent back pain. Some pillows, like latex ones, can also help reduce neck pain. Stretching before bed and after waking up can help keep muscles relaxed and reduce pain.

Drugs and diet

See also: Alertness § Drugs used to increase alertness

Drugs that help people fall asleep, called hypnotics, include benzodiazepines, nonbenzodiazepine options like eszopiclone, zaleplon, and zolpidem, as well as antihistamines such as diphenhydramine and doxylamine. Alcohol (ethanol) and melatonin can also change sleep. Some opioids like morphine and codeine may make you sleepy but can change sleep patterns.

Stimulants such as caffeine, cocaine, and methylphenidate can make it harder to sleep. Too much caffeine can stop sleep and leave you very tired the next day.

Diet

What you eat can change how well you sleep. Eating more carbohydrate-rich foods may help you fall asleep faster and stay asleep longer. A balanced diet with fresh fruits, vegetables, whole grains, and less saturated fat can make sleep better. Some studies say that drinking tart cherry juice might help with sleep problems. More research is needed to know how food affects sleep.

In culture

Sleep patterns look different in various cultures. In places without artificial light, such as before electric lights, people often slept in two periods during the night. They might go to sleep after sunset, wake up for a while, and sleep again. This pattern is called "segmented sleep." With artificial light, sleep patterns changed. People started sleeping in one longer period instead of split sleep.

Sleep appears in many stories and myths. In Greek mythology, Sleep (Hypnos) and Death (Thanatos) were brothers, children of the goddess of night (Nyx). Some famous tales include the story of Epimenides of Knossos, who slept for many years, and the Christian legend of the Seven Sleepers of Ephesus, who slept for a very long time. Another well-known story is "Rip Van Winkle" by Washington Irving, about a man who sleeps for many years and wakes up after the American Revolution.