What is Sleep?

Sleep is a state where both the body and mind take a break. During this time, muscle activity slows down, awareness of the surroundings decreases, and the body reacts less to outside signals. This reduced response to stimuli is one of the main differences between being awake and being asleep. In this guide, we will mainly look at sleep as it relates to humans.

Sleep Cycles

A sleep cycle is the regular pattern of moving between non-REM (non-rapid eye movement) sleep and REM (rapid eye movement) sleep. Both of these stages will be explained below.

Sleep timing is controlled mostly by signals from the body’s circadian clock. This clock works in a daily rhythm and usually follows outside cues like day and night. However, it can still keep running even if those signals are removed. A clear example of this is jet lag, where traveling long distances quickly can confuse the body’s natural rhythm, making it feel like the time is either earlier or later than usual.

Although our sleep ideally follows the circadian clock, many factors can affect it. These include exposure to light, work or social schedules, naps, genetics, and other personal habits.

REM and Non-REM Sleep

When we sleep, the brain uses much less energy compared to when we are awake, especially in non-REM sleep. REM sleep, on the other hand, is quite different. It is marked by rapid eye movement, temporary paralysis of most muscles, and the experience of dreams.

A complete sleep cycle lasts about 90 minutes on average, and during a good night’s sleep, this cycle repeats 4–6 times. Sleep begins with non-REM sleep, which later enters slow-wave sleep (deep sleep). In this stage, body temperature and heart rate drop, and the brain conserves energy by restoring a molecule called ATP (adenosine triphosphate), which is important for storing and transporting energy. Slow-wave sleep also releases growth hormone, which plays a vital role in human development.

REM sleep makes up a smaller share of total sleep time but is best known as the stage where most dreams and nightmares occur. If a person is deprived of REM sleep, they may experience mood problems such as anxiety, irritability, hallucinations, or poor focus. When REM sleep is restricted, the body tries to make up for it later by spending more time in this stage, a process called REM rebound. While REM sleep is widely considered important, its full role and necessity are not completely understood. Interestingly, in certain cases, missing REM sleep may cause short-term positive effects.

Sleep Quality

Sleep quality refers to how easily a person can fall asleep, and how many times they wake up during the night. It can also be measured by how refreshed someone feels when they wake up in the morning.

Poor sleep quality can interrupt the smooth flow between sleep stages and disturb the sleep cycle. For high-quality sleep, the body’s need for rest must align with the circadian rhythm. Ideally, the peak release of melatonin (the sleep hormone) and the lowest body temperature should fall after the middle of the sleep period and before waking up.

How Much Sleep Do We Need?

Sleep is still not fully understood, and the amount of rest a person requires can vary depending on age, lifestyle, and individual needs. Generally, a person who sleeps enough should not feel sleepy or tired during the day.

Research suggests that 6–7 hours of sleep per night is linked with many health benefits. However, other factors like health, stress, and environment also play an important role.

As people get older, they usually need less sleep. Newborns spend most of the day sleeping, while adults require much less. By around the age of 5, children’s sleep needs start to become closer to those of adults.

Recommended Sleep by Age (CDC Guidelines)