Sleep

Sleep is a special state where our minds and bodies rest and recover. While we are asleep, our brain stays active 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 restore its energy supplies. Sleep also makes us less aware of sounds and other things around us, though we can still notice 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 cycles between these types about every 90 minutes.

Timing

Sleep timing is controlled by the circadian clock (Process C), sleep-wake homeostasis (Process S), and to some extent by the individual will.

Sleep timing depends greatly on hormonal signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism's environment to recreate an internal day–night rhythm. Process C counteracts the homeostatic drive for sleep during the day (in diurnal animals) and augments it at night. The suprachiasmatic nucleus (SCN), a brain area directly above the optic chiasm, is presently considered the most important nexus for this process; however, secondary clock systems have been found throughout the body.

Generally speaking, the longer an organism is awake, the more it feels a need to sleep ("sleep debt"). This driver of sleep is referred to as Process S. The balance between sleeping and waking is regulated by a process called homeostasis. Induced or perceived lack of sleep is called sleep deprivation.

Humans are also influenced by aspects of social time, such as the hours when other people are awake, the hours when work is required, the time on clocks, etc. Time zones, standard times used to unify the timing for people in the same area, correspond only approximately to the natural rising and setting of the sun. An extreme example of the approximate nature of time zones is China, a country which used to span five time zones and now officially uses only one (UTC+8).

In polyphasic sleep, an organism sleeps several times in a 24-hour cycle, whereas in monophasic sleep this occurs all at once. Under experimental conditions, humans tend to alternate more frequently between sleep and wakefulness (i.e., exhibit more polyphasic sleep) if they have nothing better to do. Given a 14-hour period of darkness in experimental conditions, humans tended towards bimodal sleep, with two sleep periods concentrated at the beginning and at the end of the dark time. Bimodal sleep in humans was more common before the Industrial Revolution.

Different characteristic sleep patterns, such as the familiarly so-called "early bird" and "night owl", are called chronotypes. Genetics and sex have some influence on chronotype, but so do habits. Chronotype is also liable to change over the course of a person's lifetime. Seven-year-olds are better disposed to wake up early in the morning than are fifteen-year-olds. Chronotypes far outside the normal range are called circadian rhythm sleep disorders.

Naps are short periods of sleep that one might take during the daytime, often in order to get the necessary amount of rest. Napping is often associated with childhood, but around one-third of American adults partake in it daily. The optimal nap duration is around 10–20 minutes, as researchers have proven that it takes at least 30 minutes to enter slow-wave sleep, the deepest period of sleep. Napping too long and entering the slow wave cycles can make it difficult to awake from the nap and leave one feeling unrested. This period of drowsiness is called sleep inertia.

Ideal duration

Human 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, but many things can affect this.

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 and get ready for healthy sleep habits later in life.

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 plays important roles in keeping our bodies and minds healthy. During sleep, the brain clears out waste products and repairs itself, helped by a special system called the glymphatic system. Sleep also helps our bodies restore themselves and is especially important for the brain. It allows the brain to process and store memories better, especially during certain stages like slow-wave sleep and REM sleep.

When we sleep, especially during REM sleep, we often dream. Dreams can feel very real while we’re having them, even if they seem strange when we wake up. Some people even keep dream journals to help 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, with many adults experiencing it sometimes and about 10–15% having it regularly. Causes can include stress, a noisy or bright room, irregular sleep times, or too much excitement or activity before bed. To help with insomnia, it’s useful to keep a regular sleep schedule, avoid stressful or exciting activities before bedtime, and limit caffeine. Making the bedroom a calm place without screens or work can also help.

Exercise often helps improve sleep, especially if done several hours before bedtime. While some sleep medicines exist, they should be used carefully. Using white noise, like the sound of rain or fans, can also help people fall asleep better.

Sleep health

Sleep duration refers to how long you sleep, while sleep quality includes how quickly you fall asleep and whether you stay asleep without waking up. Good quality sleep helps you feel better and lets you handle your emotions well.

Not getting enough sleep or having poor quality sleep can lead to health problems like heart disease, being overweight, and mental health issues. Both children and adults can have trouble with sleep, and this can affect their overall health. Good sleep habits, such as going to bed and waking up at the same time each day, avoiding screens before bed, and keeping a comfortable sleep environment, can help improve sleep quality.

Sleep and musculoskeletal health

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

Mattresses that are medium-firm to firm are often recommended to help prevent back pain. Some pillows, like latex ones, can also help reduce neck pain. Stretching before bed and after waking up can also 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 affect sleep. Some opioids like morphine and codeine may induce sleep but can change sleep patterns.

Stimulants such as caffeine, cocaine, and methylphenidate can make it harder to sleep. Too much caffeine can disrupt sleep and lead to feeling very tired the next day.

Diet

What you eat can influence 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 improve sleep quality. Some studies suggest that drinking tart cherry juice might help with sleep problems. More research is needed to fully understand how diet affects sleep.

In culture

Research shows that sleep patterns differ a lot between cultures. In places without artificial light, such as before electric lights were common, people often slept in two main periods during the night. They might go to sleep soon after sunset, wake up for a while, and then sleep again. This pattern is called "segmented sleep." With the introduction of artificial light, especially in the 19th century, sleep patterns changed. People began to sleep in one longer period at night rather than 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 ancient Greek story of Epimenides of Knossos, who slept for fifty-seven years, and the Christian legend of the Seven Sleepers of Ephesus, who slept for 360 years. Another well-known story is "Rip Van Winkle" by Washington Irving, about a man who sleeps for twenty years and wakes up after the American Revolution.