We spend roughly a third of our lives asleep. For a long time, sleep seemed like a simple pause, a period when the body and brain switch off to rest. Modern science shows something very different. During sleep, the brain cycles through distinct stages, consolidates memories, clears waste products, regulates hormones and prepares the body for the next day. Going without enough sleep affects mood, attention, decision making and long-term health. This explainer describes what happens during sleep, how the body decides when to sleep and why sleep is so important. It explains how the body works and is not medical advice; persistent sleep problems are worth discussing with a doctor.
How the body decides when to sleep
Two main systems work together to control when we feel sleepy and when we feel alert.
The circadian rhythm
The circadian rhythm is the body's internal clock, running on a cycle of roughly 24 hours. The word comes from Latin "circa diem," meaning "about a day." The master clock is located in a small region of the brain called the suprachiasmatic nucleus, in the hypothalamus. It coordinates clocks found in nearly every organ and cell of the body.
The strongest signal that keeps this clock synchronized is light. Light entering the eyes, especially blue-rich daylight, sends signals to the master clock. In the evening, as light fades, the brain releases the hormone melatonin, which signals to the body that it is night and helps prepare it for sleep. In the morning, light suppresses melatonin and promotes wakefulness.
The circadian rhythm also influences body temperature, hormone release, digestion and alertness throughout the day. Body temperature, for example, typically falls in the evening and reaches its lowest point in the early morning hours.
Sleep pressure
The second system is sleep pressure, or homeostatic sleep drive. The longer you stay awake, the stronger your need for sleep becomes. A chemical called adenosine builds up in the brain during waking hours and promotes sleepiness. During sleep, adenosine levels fall. Caffeine works by blocking adenosine receptors, temporarily reducing the feeling of sleepiness without removing the underlying need for sleep.
When the circadian rhythm signals night and sleep pressure is high, we fall asleep easily. When they are out of sync, as with jet lag or shift work, sleep becomes difficult.
The stages of sleep
Sleep is not uniform. Throughout the night, the brain cycles through different stages, each with characteristic patterns of brain waves, measured by electroencephalography (EEG), as well as eye movements and muscle activity.
Sleep is divided into non-rapid eye movement (NREM) sleep, which has three stages, and rapid eye movement (REM) sleep.
| Stage | Description | Brain activity |
|---|---|---|
| N1 | light sleep, transition from wakefulness; easily awakened; may experience sudden muscle jerks | slower waves than wakefulness |
| N2 | deeper light sleep; heart rate and temperature drop; most of total sleep time | sleep spindles and K-complexes |
| N3 | deep sleep, also called slow-wave sleep; hardest to wake from | slow, large delta waves |
| REM | vivid dreams; eyes move rapidly; most muscles temporarily paralyzed | active, similar to wakefulness |
Sleep cycles
A full cycle through these stages takes about 90 minutes, and most adults go through four to six cycles per night. The balance of stages changes as the night progresses. Early cycles contain more deep N3 sleep, while later cycles, toward the morning, contain more REM sleep.
What happens during deep sleep
Deep, slow-wave sleep appears to be particularly important for physical restoration and memory:
- Growth hormone is released in large amounts, supporting tissue repair and growth, especially in children and adolescents.
- Memory consolidation: During deep sleep, the brain appears to replay and strengthen memories of facts and experiences, transferring them from short-term storage in the hippocampus to long-term storage in the cortex.
- Brain cleaning: Research in animals suggests that during sleep, the brain's glymphatic system increases the flow of fluid that clears waste products, including proteins associated with neurodegenerative diseases. How this works in humans is still being studied.
- Energy restoration: Deep sleep may help restore energy reserves in brain cells.
What happens during REM sleep
REM sleep is a paradoxical state. The brain is highly active, almost as active as when awake, but most voluntary muscles are temporarily paralyzed, preventing us from acting out our dreams. The eyes dart back and forth beneath the eyelids.
REM sleep is associated with:
- Vivid dreaming, although dreams can occur in other stages too.
- Emotional processing: REM sleep may help process emotional experiences and regulate mood.
- Learning and creativity: REM sleep appears to help integrate new information with existing knowledge and support problem solving and skills learning.
- Brain development: Newborns spend about half of their sleep in REM, which may support brain development.
Why we dream
Scientists still debate why we dream. Several theories exist:
- Dreams may help process emotions and experiences.
- They may be a side effect of the brain's activity during REM sleep, which the mind turns into stories.
- They may help simulate threats or social situations, preparing us for real life.
- They may play a role in memory consolidation and creativity.
Most people dream several times a night, but they remember dreams only if they wake during or shortly after them.
How much sleep do we need?
Sleep needs change with age. General recommendations from sleep medicine organizations are approximately:
| Age group | Recommended sleep per 24 hours |
|---|---|
| Newborns (0 to 3 months) | 14 to 17 hours |
| Infants (4 to 12 months) | 12 to 16 hours, including naps |
| Toddlers (1 to 2 years) | 11 to 14 hours, including naps |
| Preschoolers (3 to 5 years) | 10 to 13 hours, including naps |
| School-age children (6 to 12 years) | 9 to 12 hours |
| Teenagers (13 to 18 years) | 8 to 10 hours |
| Adults | 7 or more hours |
Individual needs vary somewhat, but very few people function well on less than about seven hours of sleep regularly.
Teenagers often naturally shift their body clocks later, making it hard for them to fall asleep early and wake early. This is one reason some scientists recommend later school start times for adolescents.
What happens when we do not sleep enough
Lack of sleep affects the body and mind in many ways:
- Attention and reaction time: Sleep deprivation impairs concentration and slows reactions. Drowsy driving is a major cause of road accidents.
- Mood: Lack of sleep is associated with irritability, anxiety and low mood.
- Memory and learning: Without enough sleep, it is harder to learn new information and remember it.
- Immune function: Sleep supports the immune system; chronic sleep loss is linked to a higher risk of infections.
- Metabolism: Short sleep is associated with changes in appetite hormones and a higher risk of weight gain and type 2 diabetes.
- Heart health: Long-term insufficient sleep is associated with high blood pressure and cardiovascular disease.
The modern sleep challenge
Modern life often works against good sleep. Artificial light in the evening, especially from screens, can delay the release of melatonin and shift the body clock. Irregular schedules, caffeine late in the day, stress and noise also interfere with sleep.
Sleep scientists recommend some general habits, often called sleep hygiene:
- Keep a regular sleep schedule, even on weekends.
- Get bright light in the morning and dim lights in the evening.
- Limit caffeine in the afternoon and evening.
- Keep the bedroom cool, dark and quiet.
- Avoid screens and stimulating activities close to bedtime.
- Avoid heavy meals and alcohol before bed; alcohol may help you fall asleep but disrupts sleep later in the night.
Sleep disorders
Many people experience sleep problems at some point. Common sleep disorders include insomnia, difficulty falling or staying asleep; obstructive sleep apnea, in which breathing repeatedly stops and starts during sleep; restless legs syndrome; and narcolepsy, which causes excessive daytime sleepiness. These conditions can be diagnosed and treated, so persistent problems with sleep or daytime tiredness are worth discussing with a healthcare professional.
Questions readers ask
Why do we sleep? Sleep supports memory, learning, emotional balance, immune function, metabolism and brain maintenance. Scientists are still discovering all of its functions.
What is REM sleep? A stage of sleep with rapid eye movements, vivid dreams, active brain waves and temporary muscle paralysis.
What controls when we feel sleepy? The circadian rhythm, the body's internal clock, and sleep pressure, which builds up the longer we are awake.
Does caffeine replace sleep? No. Caffeine blocks the feeling of sleepiness temporarily, but it does not provide the benefits of sleep.
How many sleep cycles do we have each night? Usually four to six cycles of about 90 minutes each.
In short
Sleep is an active, essential process. The circadian clock, guided by light and melatonin, and sleep pressure, driven by adenosine, determine when we feel tired. Each night, we cycle through light sleep, deep slow-wave sleep and REM sleep, with deep sleep dominating early in the night and REM sleep later. During these stages, the brain consolidates memories, processes emotions, clears waste and supports growth and repair. Regular, sufficient sleep is one of the foundations of health, and understanding how it works can help us protect it.