How Sleep Works: Stages, Deep Sleep, Body Clock

Sleep is a cycle, not a switch

When you sleep, you cycle through two phases: non-REM sleep and rapid eye movement (REM) sleep. The cycle starts over every 80 to 100 minutes, and there are usually four to six cycles a night. You may wake briefly between cycles. [1]

Deep sleep

Deep sleep is the brain-wave pattern of slow-wave sleep. The National Institute on Aging notes that you need deep sleep to feel refreshed in the morning. [2] The amount of slow-wave sleep peaks in early childhood, drops sharply in the teenage years, and keeps decreasing through adulthood; older people may have none at all. [1] That means less deep sleep with age is expected, not automatically a disorder. Deep sleep is also the stage from which sleepwalking arises, see Parasomnias. [3]

REM sleep

Dreaming usually happens in REM sleep, and brain activity then is close to waking activity. [1] Both non-REM and REM sleep help you store memories. [2] When the usual muscle limpness in REM fails and people act out dreams, it is called REM sleep behavior disorder, which can be dangerous. [2]

The body clock and sleep pressure

Two systems work together. The first is the circadian rhythm, the roughly 24-hour internal clock. Light and dark have the biggest influence on it, and the brain's master clock controls production of the hormone melatonin based on the light your eyes receive. [4] The second is sleep-wake homeostasis, the body's drive to sleep, which increases the longer you are awake. A chemical called adenosine builds up in the brain while you are awake and is broken down during sleep. [2] This drive is what people often call sleep pressure.

Practical meaning: regular light exposure and regular wake times support the clock, while long naps or sleeping in late can reduce the pressure that helps you fall asleep. See Sleep hygiene.

How much sleep do I need?

Experts recommend that adults sleep between 7 and 9 hours a night. Adults who sleep less than 7 hours may have more health issues than those who sleep 7 or more. [5] Teens aged 13 to 18 are advised 8 to 10 hours, and children 6 to 12 years 9 to 12 hours. [5] Needs differ between people, so how you feel in the day matters too. [6]

Sleep debt and sleep deficiency

The NHLBI uses the term sleep deficiency for getting too little sleep, sleeping at the wrong time of day, not getting the types of sleep the body needs, or having a sleep disorder that prevents enough sleep. [7] The popular phrase "sleep debt" is not defined in the sources reviewed for this page, and Proper evidence not found on how many nights it takes to repay. What the NHLBI does say is that research does not support the myth that people can learn to get by on little sleep with no negative effects. [7]

Chronotype: morning or night person

Genes may affect whether you are a deep or light sleeper. [8] Proper evidence not found in the health-agency sources read for this page that defines or measures chronotype ("morning lark" or "night owl") for the general public, so this page does not give chronotype advice. When the preferred sleep time is much later than society allows, see Body-clock problems.

How sleep changes with age

Older adults tend to go to bed earlier and wake earlier, sleep is shorter and lighter, and they may wake more often. [2] See Sleep at different life stages.

Sleep trackers: what they can and cannot do

Consumer sleep technologies are apps and devices that claim to measure or improve sleep. The American Academy of Sleep Medicine says they lack validation against the gold-standard sleep study and FDA clearance, so they cannot be used to diagnose or treat sleep disorders. They may still help start a conversation with a clinician. [9] In practice: treat stage numbers from a wearable as rough estimates, and talk to a clinician if your tracker shows something that worries you or you feel unwell during the day. For sleep studies that clinics do use, see Sleep apnea.

When to get help

MedlinePlus lists these as signs of a possible sleep disorder: regularly taking more than 30 minutes to fall asleep, often feeling sleepy or falling asleep at the wrong times in the day, and a partner noticing loud snoring, gasping or pauses in breathing. [10] See How to fall asleep faster for first steps.