Sleep as an active state
Sleep can look like a pause, but measurements of brain activity show that it is an organized and changing process. Scientists study it with tools such as electroencephalography (EEG), which records electrical activity at the scalp, along with measures of eye movement, muscle tone and heart rate. Taken together, these recordings reveal a pattern of stages that repeats through the night. Researchers do not yet agree on every detail of why we sleep, but the broad outline described here is widely accepted.
Sleep stages and sleep cycles
Sleep is generally divided into non-REM (NREM) sleep and REM sleep.
NREM sleep has three stages in the standard scheme:
- Stage 1. A brief, light stage right after falling asleep. People are easily woken and may feel they have not slept at all.
- Stage 2. Light sleep with slower brain activity and occasional short bursts of activity called sleep spindles. Heart rate and body temperature drop. This stage fills a large part of the night.
- Stage 3. Deep or slow-wave sleep, with large, slow brain waves. It is hard to wake someone from this stage, and they may feel groggy if woken.
REM sleep (rapid eye movement) shows brain activity that looks closer to waking, with fast eye movements and vivid dreams. Most of the body’s voluntary muscles are temporarily relaxed, which is thought to keep people from acting out dreams.
These stages come in sleep cycles that run roughly 90 minutes, though cycle length differs among people and across the night. Early cycles tend to contain more deep sleep, and later cycles more REM sleep. People briefly surface between cycles and usually do not remember it. This is why waking up at the end of a cycle is sometimes said to feel easier, although the practical benefit of timing alarms this way is uncertain, since cycle lengths vary and are hard to know without measurement.
The body clock
Why do we feel sleepy at night and alert in the morning? Two main processes are generally described.
The circadian rhythm. Humans have an internal clock, centered in a small region of the brain, that runs on a cycle of roughly 24 hours. Daylight is the strongest cue that keeps it aligned with the outside day. Body temperature, hormones and alertness all follow daily patterns shaped by it. Melatonin, a hormone that tends to rise in the evening in dim light, is often described as a signal of biological night, not as a simple on switch for sleep.
Sleep pressure. The longer a person is awake, the stronger the drive to sleep tends to become. Researchers link this to chemical changes in the brain, with adenosine often cited as one candidate, and caffeine is thought to work in part by blocking its effects. Sleep reduces the pressure again.
Normally the two processes work together: pressure builds across the day while the clock timing makes the evening favorable for sleep. When they fall out of step, as in shift work or after crossing time zones, sleep can be disrupted, which is part of why jet lag feels the way it does. Also, people differ in their natural timing. Some tend toward evenings, others toward mornings, and teenagers often shift later, as a normal part of development.
Why does rest matter?
No single explanation covers everything, and scientists generally think sleep serves several purposes. Ideas with reasonable support include:
- Memory and learning. Sleep appears to help stabilize and organize what has been learned. The details are covered in our piece on sleep and cognition.
- Brain maintenance. Some research, mostly in animals, suggests sleep may help clear waste products from the brain. This is an active area, and how far it applies to humans is still being studied.
- Physical restoration. Growth and repair processes are thought to be more active during sleep, particularly deep sleep.
- Emotional regulation. After poor sleep, people often feel more irritable and reactive. REM sleep is of interest to researchers studying emotions.
- Energy and metabolism. Sleep is linked to how the body handles appetite hormones and blood sugar, and short sleep has been associated with changes in both.
These ideas are not mutually exclusive.
Sleep across life
Sleep changes with age. Newborns sleep for most of the day in short stretches, children need more sleep than adults, and many older adults find that sleep becomes lighter and more fragmented. Typical adult needs are often given as seven to nine hours, with individual variation. The practical, habit-based side of this is covered in our article on sleep hygiene.
What goes wrong, and when to get help
Common sleep problems include insomnia, sleep apnea (repeated pauses in breathing) and restless legs. They are often treatable. It is worth speaking to a doctor if poor sleep persists, if you snore loudly with breathing pauses, or if you are very sleepy during the day despite enough time in bed. This article is general information and not a substitute for medical advice.
Open questions
Despite decades of research, there is still debate about exactly why sleep evolved, how much sleep is ideal for each person, and how some findings in animals translate to humans. A careful reading of the evidence supports a modest conclusion: sleep is a regulated, structured and essential process, and regularly getting enough of it is associated with better functioning and health.