Two systems control sleep. Sleep pressure builds the longer you are awake, partly as a brain chemical called adenosine accumulates. Your circadian body clock, set mainly by light, decides when you feel sleepy or alert. When pressure is high and the clock says night, you fall asleep. Your brain then cycles through non-REM and REM sleep about four to six times before morning.
Scientists call this the two-process model of sleep regulation (Borbely and colleagues, 2016; NINDS). For why we dream and how to remember dreams, see dreams explained.
This page is the overview: the whole night, end to end, in plain language. Each section points to the page or calculator that goes deeper.
What Is Sleep?
Sleep is a period of rest that alternates with wakefulness. Internal body clocks of roughly 24 hours control when you are awake and when your body is ready for sleep. Once you are asleep, you move through the stages in a predictable pattern (NHLBI).
You spend about a third of your life asleep. NINDS describes quality sleep at the right times as essential to survival, like food and water (NINDS).
Sleep is not a switched-off brain. Your brain and body stay remarkably active while you sleep. Several brain areas share the work (NINDS):
- The hypothalamus and its suprachiasmatic nucleus, the master clock.
- The brainstem, which controls the switch between waking and sleep.
- The thalamus.
- The pineal gland, which makes melatonin.
- The basal forebrain.
Within about a minute of falling asleep, body temperature drops and brain activity ramps down. Heart rate and breathing slow, and the body uses less energy (Sleep Foundation).
The Two-Process Model: Sleep Pressure And The Body Clock
The two-process model dates from the early 1980s and is still a major framework in sleep research. A homeostatic process (Process S) is sleep pressure. A process controlled by the circadian pacemaker (Process C) is the body clock. Together they explain both the timing and the intensity of sleep (Borbely and colleagues, 2016).
| Process S: sleep pressure | Process C: circadian clock | |
|---|---|---|
| What it tracks | How long you have been awake | The time of day inside your body |
| Main driver | Adenosine rising in the brain while you are awake | Light reaching the eyes, relayed to the suprachiasmatic nucleus |
| What resets it | Sleep: pressure falls as you sleep | Daylight, darkness and regular sleep and meal times |
| Key chemicals | Adenosine (blocked by caffeine) | Melatonin in the evening, cortisol in the morning |
| What throws it off | Caffeine late in the day | Evening light, jet lag, night shifts |
Sources: NHLBI, NINDS, Sleep Foundation.
Sleep Pressure And Adenosine
Your body has a biological need for sleep, and it grows the longer you are awake. A compound called adenosine is linked to this need. Its level in the brain keeps rising while you are awake, and the rise signals a shift toward sleep (NHLBI).
This sleep drive gets stronger every hour you are awake. After a period without sleep, it makes you sleep longer and more deeply (NINDS).
Adenosine is a by-product of the brain using energy. Scientists think that staying awake too long makes it limit activity in brain areas that keep you alert. Once you are asleep, the brain converts adenosine back, which clears the drive by morning (Sleep Foundation).
Caffeine works mainly by blocking adenosine receptors. That is why coffee hides sleepiness without removing the underlying need (Sleep Foundation; NINDS).
See how your afternoon coffee could still be working at bedtime with the caffeine impact on sleep calculator. If you have been short of sleep for days, the sleep debt calculator adds up how much pressure you are carrying.
The Circadian Clock
Your central circadian clock sits in the brain and tells you when it is time to sleep. Other clocks run in organs throughout the body. Light, darkness and other cues keep them in step with the day.
Most people’s natural cycle is slightly longer than 24 hours, so the clock needs daily resetting. Artificial light and caffeine can disrupt it by sending false wakefulness signals (NHLBI).
The suprachiasmatic nucleus receives information about light directly from the eyes (NINDS). As evening light fades, melatonin starts to rise. It peaks in the early morning. As light returns, the body releases cortisol, which prepares you to wake.
Bright artificial light late in the evening can stop the brain from releasing melatonin. That makes it harder to fall asleep (NHLBI).
How The Two Processes Work Together
If sleep pressure worked alone, you would feel most alert in the morning and steadily worse all day. The clock prevents that. It pushes alertness up during the day to counter rising pressure. That is why you can feel as sharp at 4 p.m. as at 10 a.m.
On average, people feel most tired just after midnight and during the early-afternoon slump (Sleep Foundation). Sleep comes most easily when both systems line up: high pressure and a clock that says night.
Find where your own clock sits with the circadian rhythm calculator, which estimates your body-clock timing from your usual sleep times.
What Happens Across One Night
Once you fall asleep, you cycle through two phases: non-REM and REM sleep. A cycle starts over every 80 to 100 minutes. There are usually four to six cycles per night, and you may wake briefly between them (NHLBI). Here is the night at overview level:
- Falling asleep (N1). A short changeover lasting several minutes. Heartbeat, breathing and eye movements slow, and brain waves begin to slow (NINDS).
- Light sleep (N2). Body temperature drops and eye movements stop. You spend more of your cycles in this stage than any other (NINDS).
- Deep sleep (N3). Slow-wave sleep, the stage you need to feel refreshed. It comes in longer periods during the first half of the night. It can be hard to wake you from it (NINDS).
- REM sleep. First arrives about 90 minutes after you fall asleep. Brain activity is closer to waking, and the eyes move rapidly. Arm and leg muscles are temporarily paralyzed so you do not act out dreams. REM periods get longer and deeper later in the night (NINDS).
The overall shape of the night is therefore front-loaded with deep sleep and back-loaded with REM sleep (NHLBI).
During most stages the thalamus goes quiet, so you can tune out the world. In REM sleep it becomes active. It sends the cortex the images, sounds and sensations that fill dreams (NINDS).
For more detail, use the deep pages rather than this overview:
- What each stage does to your brain and body, and how to get more deep sleep: stages of sleep.
- How long a cycle really lasts and why 90 minutes is only an average: sleep cycle science.
- Which stage you dream in and why dreams change through the night: how sleep cycles affect your dreams.
Curious how your own night might divide up? The REM sleep calculator estimates time in light, deep and REM sleep from typical values. To time your alarm around whole cycles, start with the sleep calculator.
Why Do We Need Sleep?
Everyone needs sleep, but its exact biological purpose is still a mystery (NINDS). What is well established is how widely it acts.
Sleep affects almost every tissue and system in the body. That includes the brain, heart and lungs, metabolism, immune function, mood and disease resistance (NINDS). The main jobs that NINDS and NHLBI describe are:
- Learning and memory. Without sleep you cannot form or maintain the brain pathways that let you learn and create new memories (NINDS). Sleep helps form long-term memories. Too little makes it harder to focus and think clearly (NHLBI).
- Brain housekeeping. Recent findings suggest sleep removes toxins that build up in the brain while you are awake (NINDS).
- Heart and circulation. In non-REM sleep, blood pressure and heart rate fall. The heart does not work as hard as it does when you are awake (NHLBI).
- Hormones and metabolism. Too little quality sleep is linked to higher levels of the hunger hormones leptin and ghrelin. It is also linked to a weaker response to insulin and to eating more fatty, sweet and salty food (NHLBI).
- Immunity. Some immune cells work harder while you sleep. That is one reason people who do not sleep enough may catch more colds and other infections (NHLBI).
- Growth. In children and teens, sleep also supports growth and development (NHLBI).
Over time, a chronic lack of sleep or poor-quality sleep raises the risk of high blood pressure, cardiovascular disease, diabetes, depression and obesity (NINDS). Adults are advised to sleep 7 to 9 hours a night. Adults who regularly get less than 7 hours may have more health problems (NHLBI).
How Sleep Changes With Age
Both how much you sleep and how your sleep is built change across life. People of the same age vary a lot, and there is no single right amount for everyone (NINDS).
| Life stage | Recommended sleep per day | What changes in how you sleep |
|---|---|---|
| Newborns (under 4 months) | Patterns vary widely; babies may sleep 16 to 18 hours | More time in REM sleep; no settled body clock yet, which develops around 4 to 6 months |
| Babies 4 to 12 months | 12 to 16 hours, including naps | Sleep gathers into longer blocks as the circadian rhythm forms |
| Children 1 to 12 years | 11 to 14 hours (age 1 to 2), 10 to 13 (3 to 5), 9 to 12 (6 to 12) | Deep slow-wave sleep peaks in early childhood |
| Teens 13 to 18 | 8 to 10 hours | Deep sleep drops sharply; the clock shifts later, so later bedtimes feel natural |
| Adults | 7 to 9 hours | Deep sleep keeps declining slowly; less time in REM sleep with age |
| Older adults | 7 to 9 hours is still advised | Lighter, more broken sleep, earlier waking, about 3 to 4 awakenings a night |
Sources: NHLBI recommended hours, NHLBI sleep stages, NHLBI sleep/wake cycle, NINDS, Sleep Foundation, MedlinePlus.
In older adults, total sleep time stays the same or drops slightly, to around 6.5 to 7 hours. Falling asleep often takes longer and waking is more frequent, because less time is spent in deep sleep (MedlinePlus).
Brain cells that promote sleep are lost as part of normal aging. Less activity and less time outdoors also weaken the clock. So older adults usually sleep less and wake earlier (NHLBI).
Slow-wave sleep continues to fall through adulthood. Some older people may have none at all (NHLBI).
Check the recommended range for your age with the sleep calculator by age and gender.
How Is Sleep Measured?
The reference test is polysomnography, an overnight sleep study. You cannot feel sleep stages. They are defined by measurements of brain activity and eye movements recorded with sensors (NHLBI).
- What it records. A polysomnogram records breathing, oxygen levels, eye and limb movements, heart rate and brain waves through the night. Your sleep is usually captured on video. The data shows a sleep specialist whether you are reaching and moving through the stages normally (NINDS).
- How it is done. A technician places electrodes on your chin, scalp and the outer edge of your eyelids. Monitors for heart rate and breathing go on your chest. The test measures how long it takes you to fall asleep and to reach REM sleep (MedlinePlus). An EEG tracks the brain waves that mark each stage (Sleep Foundation).
- Home tests. If the main concern is sleep apnea, a home study may be used instead (MedlinePlus). Home apnea tests collect breathing, heart rate and other data. They give less information than a full study and are not watched by a technologist (Sleep Foundation).
- Wearables and apps. Phones, watches, bands and bedside monitors can record sound, movement, hours slept, heartbeat and breathing. NINDS describes this as an informal way of collecting sleep data (NINDS). Treat the stage breakdown on a tracker as an estimate, not a sleep study. Our guide to sleep tracking and insomnia covers when a nightly score helps and when it adds worry.
What Can Push The System Off Balance?
Many things affect your sleep-wake needs. They include medical conditions, medicines, stress, the sleep environment, age, and what you eat and drink. Perhaps the greatest influence is light (NINDS).
In two-process terms, most problems either weaken sleep pressure at bedtime or put the body clock out of step with the wall clock.
Keeping regular sleep and wake times, allowing 7 or more hours in bed and timing meals and caffeine sensibly are the simplest ways to keep both systems working together (Sleep Foundation).
When To Talk To A Doctor
See a doctor if you have problems sleeping or feel unusually tired during the day. Most sleep disorders can be treated effectively (NINDS). Daytime sleepiness, or holding your breath in sleep followed by gasps or snorts, are reasons a doctor may suggest a sleep study (MedlinePlus). Our checklist of signs you have a sleep disorder can help you decide what to raise.
How Sleep Works FAQs
What Controls When I Feel Sleepy?
Two systems: sleep pressure, which builds the longer you are awake, and your circadian clock, which is set mainly by light.
What Is Adenosine?
A brain chemical that builds up while you are awake and adds to sleep pressure. Sleep clears it, and caffeine blocks its effect.
How Many Sleep Cycles Do You Have A Night?
Usually four to six, each lasting about 80 to 100 minutes, according to NHLBI.
Why Do I Get Sleepy After Lunch?
The body clock has a natural dip in alertness in the early afternoon, on top of the sleep pressure built up since morning.
Does Everyone Need 8 Hours Of Sleep?
No. Adults are advised to get 7 to 9 hours, and needs vary between people of the same age.
Can A Sleep Tracker Tell My Sleep Stages?
It can estimate them. Only a sleep study that records brain waves and eye movements measures stages directly.