How Sleep Works


Sherrie Neustein, M.D.
Medically Reviewed By

Sherrie Neustein, M.D.

Afy Okoye
Written By

Afy Okoye

Everyone sleeps, but not everyone knows what happens in the mind and body as they snooze. On the surface, sleep is a reversible state of rest that affects how people think and feel during the day. On closer inspection though, sleep is a complex biological process that influences nearly every cell and tissue in the body.

Alternating periods of sleep and wakefulness, called the sleep-wake cycle, is regulated by the brain. During sleep, the body progresses through four sleep stages. Each is necessary to get the full benefits of a night’s rest.

Sleep is critical for mental and physical health and plays a role in a wide range of biological functions, like metabolism, mood, and the immune system. Chronic sleep loss is a common issue that can increase the risk of health conditions like obesity, high blood pressure, and heart disease.

It is hard to overstate the importance of sleep. Understanding how sleep works, and how much rest you need, can help you improve your health and get the most out of your sleep each night.

Why Do We Sleep?

Researchers are still unraveling the mysteries of sleep. While science doesn’t have a clear answer for the exact purposes of sleep, research shows that sleep is vital for healthy functioning in a number of ways.

  • Better health: Sleep is a pillar of health and is just as important as diet and exercise.
  • Hormonal balance: Several important hormones are produced during sleep, including those that repair cells and muscles, support growth and development, and combat illnesses.
  • Optimal performance: Getting the right amount of quality sleep improves focus, enhances concentration, and aids in decision-making.
  • Enhanced learning and memory: Sleep supports brain plasticity, helping you learn and form long-term memories.

Sleep loss can also take a significant toll on your mind and body.

  • Health complications: Poor sleep can increase the risk for chronic health conditions like obesity, diabetes, heart disease, and kidney disease.
  • Mental fog: Lack of sleep can make it harder to focus, stay alert, and make good decisions.
  • Reduced immunity: Not getting enough sleep can affect the immune system, making you more susceptible to getting sick.
  • Mood changes: Sleep deprivation can leave you feeling more anger and frustration as well as increasing the risk of depression.

The Sleep-Wake Cycle

The sleep-wake cycle describes the body’s pattern of being asleep or awake is regulated by two internal processes: circadian rhythm and the homeostatic sleep drive.

Circadian Rhythm

Circadian rhythms are biological cycles that repeat nearly every 24 hours. These daily rhythms are controlled by internal clocks. Our internal clocks control most of what happens in our bodies, including body temperature, hormones, metabolism, and even when we feel tired or alert.

The sleep-wake cycle is an important circadian rhythm, controlled by a central clock in the brain. This part of the brain signals to the body when it is time to sleep by controlling the release of melatonin, a hormone important for feeling sleepy.

A variety of factors, both in the body and in the environment, can affect a person’s circadian rhythms, including:

  • Light: Well-timed exposure to light can help with feeling alert, but getting too much artificial light in the evening from TVs, cellphones, and other sources can suppress melatonin production and make it more difficult to fall asleep.
  • Jet lag: Traveling through different time zones can cause disruptions to circadian rhythms. Jet lag occurs when the internal clock does not match the night-day cycle of the new destination.
  • Work schedule: People who work the night shift or rotating schedules can find it challenging to stay alert during work and to fall asleep when they need to. Working shifts that disturb the sleep-wake cycle increases the risk of circadian rhythm sleep disorders.

Homeostatic Sleep Drive

The homeostatic sleep drive also influences a person’s sleep-wake cycle. Also described as sleep pressure, the homeostatic sleep drive increases the body’s need for sleep while a person is awake and decreases while a person is sleeping. Sleep pressure reaches its lowest point after a person wakes up from a full night’s rest.

In addition to time spent awake, sleep pressure can build due to mentally or physically demanding activities and while a person is sick or fighting an infection.

What Happens During Sleep?

Sleep is a dynamic period in which the body undergoes a series of changes that are vital for health. During a night of rest, a person cycles multiple times through the four sleep stages.

Sleep Stages

There are four stages of sleep. The first three sleep stages are called non-rapid eye movement (NREM) sleep and the final sleep stage is called rapid-eye-movement (REM) sleep.

Sleep stages repeat in a cycle, starting at stage 1 and ending in REM sleep. Each sleep cycle lasts about 90 to 120 minutes and people typically go through four to six sleep cycles in a full night of sleep. Each sleep stage is associated with characteristic changes in the brain and body.

Sleep Stage Sleep Category Description Length
Stage 1 NREM Light sleep that begins as a person is drifting off. Heartbeat and breathing begin to slow down as muscles relax. 1 to 7 minutes
Stage 2 NREM A transitional period as a person moves closer to deep sleep. Heart rate and breathing continue to slow and the muscles relax further. 10 to 25 minutes
Stage 3 NREM Referred to as deep sleep or slow-wave sleep, this stage is believed to be particularly important to waking up feeling refreshed. 20 to 40 minutes
Stage 4 REM During this active stage of sleep, breathing and heart rate increase. The eyes move rapidly back and forth while the body is in a temporary state of paralysis. 10 to 60 minutes

Sleep patterns, including how long a person remains in each stage of sleep, is called sleep architecture. Sleep architecture changes throughout the lifespan and is influenced by a variety of factors, including a person’s weight, medication and drug use, as well as recent sleep loss.

What are dreams?

Dreams are the mental images and sensations experienced during sleep. Most people spend about two hours dreaming each night but only remember a fraction of their dreams. Dreams can happen in any stage of sleep, but research suggests that dreams during REM sleep are more vivid, longer, and may be more story-like than those that occur during NREM sleep.

Humans have long speculated about the origin and meaning of dreams. Many researchers believe that dreams are a continuation of daytime thought processing, while others propose that dreams help the nervous system prepare for a new day. Some researchers suggest that dreams have no meaning or purpose at all.

How Much Sleep Do You Actually Need?

The amount of sleep a person needs depends on their age, genetic makeup, level of physical activity, and other individual factors. The National Sleep Foundation provides general recommendations for sleep quantity based on age. However, some people may need slightly less or more than the recommended amount to feel alert and refreshed the following day.

Age Range Daily Recommended Hours of Sleep
Newborn (0-3 months) 14 to 17 hours
Infant (4-11 months) 12 to 15 hours
Toddler (1-2 years) 11 to 14 hours
Preschooler (3-5 years) 10 to 13 hours
School Age (6-12 years) 9 to 11 hours
Teenager (13-18 years) 8 to 10 hours
Young Adult (19-25 years) 7 to 9 hours
Adult (26-64 years) 7 to 9 hours
Older Adult (Over 65 years) 7 to 8 hours

What Does Quality Sleep Look Like?

When it comes to sleep, both quantity and quality are essential. Quality sleep is consistent and restorative and depends on several factors, including how quickly a person falls asleep, how long they stay asleep, and how often they wake up during the night. Characteristics of healthy sleep include:

  • Getting enough sleep
  • Sleeping through the night
  • Waking up feeling well-rested
  • Falling asleep quickly
  • Sleeping and waking up at the same time each day
  • Maintaining alertness during the day


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