What Is Sleep? Why, When, and How Our Bodies Rest


Written by Afy Okoye

Reviewed by Dr. Michael Breus

Our Editorial Process

Table of Contents

Sleep is a fascinating and complex phenomenon that researchers are just beginning to understand. Around one-third of your life is spent sleeping and, while the need for sleep is clear, science can’t yet explain the exact purpose of nightly rest.

We cover the basics of sleep, from what happens in the body after we doze off to the complex processes that control when you sleep and when you wake up. We also discuss the benefits of sleep and give tips for improving your nightly rest.

Key Takeaway


  • Sleep is a vital natural process that allows the body and mind to rest and recover.
  • The sleep cycle consists of distinct stages, including NREM and REM sleep, each with a specific role in maintaining your health.
  • Sleep is regulated by your circadian rhythm, and its importance extends to cognitive, emotional, and physical health.
  • Allocate the recommended hours of sleep every night to ensure your body can benefit from the restorative functions of sleep.

What Happens During Sleep?

For a long time, sleep was thought of as a state in which the body stops, shuts down, and takes a break. Research hasn’t backed up this initial theory. As you close your eyes and drift off to sleep, some processes in your body slow down, while others remain surprisingly active.

Sleep begins in a portion of the brain called the hypothalamus, which begins a cascade of sleep-inducing changes in the body. Nerve cells in the brain reduce alertness and promote sleepiness, while electrical activity in the brain begins to slow. These and other changes that occur as sleep continues affect nearly every cell in your body.

What Are Sleep Cycles?

During a night of sleep, your body cycles through four stages of sleep. The first three stages are called non-rapid eye movement (NREM) sleep and the final stage is called rapid eye movement (REM) sleep. Each stage is associated with a different depth of sleep as well as specific patterns in brain waves and muscle activity in the body.

Your body cycles through all of the stages of sleep four to six times each night, going through one about once every 90 to 120 minutes. The patterns in how you progress through these sleep cycles are affected by a variety of factors, including age, sex, and recent sleep habits.

What Is Non-REM Sleep?

Up to 80% of the time you spend asleep is in non-rapid eye movement sleep. During each cycle of NREM sleep, your body transitions through three sleep stages. Each stage is associated with certain changes in the body.

  • Stage 1: The first stage of NREM sleep is a period of very light sleep that begins as you doze off and transition into sleep. As you fall asleep, your muscles begin to relax and your heart rate and breathing slow down.
  • Stage 2: The second stage of NREM sleep is another stage of light sleep. As your body moves towards sleeping more deeply, your muscles continue to relax and your heart rate and breathing continue to slow down.
  • Stage 3: Also called slow-wave sleep, the third stage of NREM sleep is deep sleep that is important for feeling well-rested in the morning. During this stage, your muscles are fully relaxed and your heart rate and breathing are at their slowest of the night.

What Is REM Sleep?

Rapid eye movement, or REM sleep, is the fourth stage of sleep. During REM sleep, brain activity increases and both heart rate and breathing become more rapid. Your eyes begin to move quickly behind closed eyelids. 

REM sleep is highly important for brain development. Infants spend up to 50% of their sleep in this stage and in adulthood research has shown REM sleep lasts longer after periods of learning.
Dr. Michael Breus

During the REM sleep stage, your body enters a temporary state of paralysis causing you to be unable to move many muscles, like those in your arms and legs. Researchers suspect that paralysis during this sleep stage keeps you from acting out your dreams in bed.

Most dreams take place during REM sleep. The purpose of REM isn’t fully understood, and researchers continue to study REM sleep. REM sleep appears to be important for learning and memory.

Processes that Regulate Sleep

Sleep is regulated by two processes in the body, circadian rhythms and sleep drive. How tired you feel, when you fall asleep, and when you wake up are all driven by the effects of these two processes.

Circadian Rhythms

Circadian rhythms are daily cycles that affect the function of every cell in the body. Circadian rhythms direct physical processes such as:

  • Sleep and wake times
  • Brain activity
  • The release of hormones 
  • Body temperature 
  • Appetite 

Circadian rhythms are coordinated by an internal biological clock called the suprachiasmatic nucleus (SCN). The SCN is a group of nerve cells in your brain that act as a command center, regulating circadian rhythms throughout the body, including your daily cycle of sleep and wakefulness.

The SCN gets input from your eyes and takes cues from your environment to sync your body with the time of day. Cues that affect your circadian rhythms include:

  • Light and darkness 
  • Temperature 
  • Physical activity
  • Eating patterns 
  • Social interactions

Light is the most important driver of circadian rhythms. Exposure to light as you’re waking up signals to the brain that it’s daytime and shifts your circadian rhythms accordingly. Conversely, exposure to light before bedtime can make it more difficult to sleep by signaling to the brain that it’s still daytime.

Sleep Drive

The second process in the body that regulates sleep is called sleep drive. The sleep drive is your body’s way of monitoring your need for sleep, helping you to sleep longer and more deeply in times of need.

Your sleep drive is at its lowest after you wake up from a full night of sleep. The body’s need for sleep then builds the longer that you’re awake. Once you fall asleep, your sleep drive begins to decline. If you miss sleep or get poor-quality sleep, your sleep drive may remain high and result in an accumulation of sleep debt.

Other factors that increase your sleep drive include sickness and mentally taxing or physically demanding activities. This is why you may feel sleepy or sleep longer when you’re ill, while traveling, or after exercise.

Why Do You Need Sleep?

Sleep is physically necessary for the proper functioning of every cell and system in the body. While scientists are only beginning to unravel the mysteries of sleep, researchers have come up with several theories to explain why you need sleep.

  • Inactivity Theory: The inactivity theory suggests that, through evolution, nighttime sleep developed as a response to an increased risk of attack from predators after dark. Those who were less active at night faced a reduced risk of being killed.
  • Energy Conservation Theory: The energy conservation theory is another theory based on evolution. Because it’s more difficult to find food at night, this theory suggests that sleeping through the night allows humans to save their energy for the daytime.
  • Brain Plasticity Theory: Brain plasticity theory suggests that sleep is needed for the development and proper function of the brain. Research supports this theory, showing that sleep is important for cognitive functions like learning and memory.
  • Restorative Theory: The restorative theory of sleep suggests that sleep is needed to feel well-rested in the morning and for the body to restore cells that are used or damaged during the day.

The restorative function of sleep is supported by research that has identified a variety of important benefits that sleep provides to the mind and body.

  • Growth and Healing: Research shows that sleep is important for tissue growth and repair. In fact, sleep may be one of the most important ways to recover from exercise or injury.
  • Immunity: Changes in the body during sleep are important for maintaining a healthy immune system. Getting enough high-quality sleep reduces the risk of infections, lessens the severity of certain diseases, and improves the body’s response to vaccines.
  • Clearing Toxins: Sleep may help the brain eliminate toxins. Research in mice suggests that sleep may temporarily increase the distance between brain cells, providing an opportunity for the body to remove toxins that accumulate during the day.

How Much Sleep Do You Need?

The amount of sleep you need depends on your age, health, lifestyle, and your recent sleep habits. Experts provide general recommendations for how much sleep you need based on age:

Age GroupAge RangeRecommended Hours of Sleep per 24 Hours
Newborns0-3 months14-17 hours
Infants4-11 months12-15 hours
Toddler1-2 years11-14 hours
Preschool3-5 years10-13 hours
School Age6-12 years9-11 hours
Teen13-18 years8-10 hours
Adult19-64 years7-9 hours
Older AdultsOver 65 years7-8 hours

Tips for Sleeping Well

Getting the right amount of high-quality sleep can help you stay healthy and perform well at work or school. Experts suggest a variety of tips to help you sleep better.

  • Develop a Routine: Establishing some bedtime rituals can help you wind down before bed and set you up for a night of restful sleep. For better sleep, try creating a consistent pattern of relaxing activities at night. That includes sticking to a consistent bedtime and waking up around the same time every day.
  • Manage Stress: Stress is a normal reaction to difficult experiences, but left unaddressed it can significantly interfere with your sleep. Finding ways to manage your daytime stress, like listening to music or doing yoga, may make it easier for you to sleep at night. 
  • Get Active: Walking or exercising each day provides a plethora of health benefits and may even improve your sleep. Try for at least 30 minutes of physical activity each day in the morning or afternoon. Avoid exercising within a few hours of bedtime, which could keep you awake.
  • Catch Some Early Light: Getting natural light early in the day may help you sleep better at night. In fact, an hour of sunlight early in the day may combat difficulties falling asleep at night. But make sure lights and electronics are turned off at night before bed, as being exposed to light too late in the evening may keep you up.
  • Nap Wisely: A well-timed nap can help you feel more alert during the day and make up for lost sleep, but napping too late in the afternoon can make it difficult to fall asleep when bedtime rolls around. Experts recommend keeping naps to around 20 minutes and avoiding naps after around 3 p.m.
  • Watch Your Food and Drinks: What you consume in the evening can affect your sleep. To improve your sleep, cut back on caffeine from coffee, colas, or teas, especially late in the day. Avoiding large meals before bed may also help to improve your sleep.
  • Cut Back on Smoking and Alcohol: Both alcohol and tobacco can interfere with sleep and keep you awake. While a drink before bed may make you feel relaxed, it can also cause you to wake up at night and reduce the amount of deep sleep and REM sleep you get.
  • Optimize Your Sleep Space: Creating the right sleep environment can help you sleep more comfortably. Whether you’re in your bedroom or a hotel, try to keep the room cool, quiet, dark, and free from distractions like a TV or cell phone. Design your bedroom as an oasis for sleep and sex, not for entertainment or stimulating activities.

Talk to your doctor if you have trouble getting sufficient, high-quality rest at night. If needed, a doctor can help to assess your sleep issues and check for conditions that can interfere with restful sleep. Doctors can also recommend strategies and treatments to improve your sleep.

About The Author

Afy Okoye

Staff Writer, Sleep Health

Afy is a writer and creative strategist in San Francisco with a master’s degree in international health policy from the London School of Economics. She has written for VeryWell Health, BlackDoctor.org, and Paste magazine and edited peer-reviewed journal manuscripts for Elsevier. Afy says her work with The Sleep Doctor is anything but “sleepy.” She enjoys the opportunity to learn new information and share knowledge that gives people the power to make better choices. Afy also likes to read non-fiction, do creative writing, and travel solo.

  • POSITION: Side Sleeper
  • TEMPERATURE: Hot Sleeper

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