How Sleep Works


Sleep is an essential function of the human body, necessary for maintaining physical and mental health. This complex biological process (1) has innumerable benefits, including helping us learn and integrate new information, allowing time for the body to repair damaged cells and tissues, and helping us to fight infections.

Getting enough sleep can help you feel more rested, become more optimistic, and improve your relationships (2). While learning about the science of sleep isn’t a requirement for getting quality rest, many people find it enlightening to learn about what happens when they fall asleep at night.

The Sleep-Wake Cycle

The sleep-wake cycle is the body’s natural rhythm of alternating periods of sleep and wakefulness (3). The brain regulates the sleep-wake cycle through an intricate system that receives input from two sources (4): the body’s sleep drive and the circadian clock.

  • Sleep Drive: The pressure to sleep builds the longer we stay awake. Once we fall asleep, sleep pressure dissipates until we wake up and the process begins again. The amount of sleep drive we accumulate also affects how long we sleep and sleep intensity (5). The sleep drive explains why we sleep longer and more deeply after periods of sleep deprivation.
  • Circadian Clock: A central clock in the human brain organizes many daily rhythms in our bodies and behaviors (6). Circadian rhythms are 24-hour cycles (7) that influence our desire to sleep during one part of the day and stay awake at another. These cycles are impacted by our environment, which is one reason why our ability to fall asleep is affected by factors like light and temperature.

Sleep Phases

When a person falls asleep, the body begins cycling through two alternating sleep phases: rapid eye movement (REM) sleep and non-rapid eye movement (non-REM) sleep. The body cycles through these phases several times a night, with each cycle lasting for around 80 to 100 minutes (8):

Non-REM Sleep

The non-REM phase of sleep is vital for memory formation. In fact, missing this phase of sleep can cause a person’s ability to learn new things to decrease by up to 40% (9). Accounting for around 80% of the time we spend asleep (10), non-REM sleep includes three stages:

  • Stage 1: The first stage of sleep lasts several minutes. During this short period of light sleep, a person’s heart rate and breathing slows as the muscles of the body relax.
  • Stage 2: A person is still transitioning to deeper sleep while in the second sleep stage. During this stage, heart rate, breathing, and muscles continue to relax. Interestingly, this stage is where we spend the most time during sleep.
  • Stage 3: In the third stage of non-REM sleep, heart rate and breathing drop to their lowest levels of the night. This sleep stage is an important time for restoring energy and allowing the body to grow and heal.

REM Sleep

The body usually enters the REM phase around 90 minutes after falling asleep. During this phase of sleep, heart rate and breathing increase to levels near those seen while a person is awake. Most muscles of the body are temporarily paralyzed during REM sleep (11), a phenomenon called atonia. Since most dreams occur during REM sleep, atonia prevents sleepers from acting out their dreams.


People spend around two hours dreaming every night. While the most vivid dreams usually occur during REM sleep, dreaming can happen during any sleep phase (12). Although some researchers propose that dreams play a role in learning and memory (13), scientists still aren’t completely sure why we dream.

How Much Sleep Do We Need?

The amount of sleep an individual needs varies from person to person. Recommended amounts of sleep also depend on a person’s age (14):

  • Babies: Newborns may sleep up to 18 hours per day , which is important for their growth and development. Infants and toddlers may sleep 11 to 16 hours a day, including nap times.
  • Children and Teens: From ages 3 to 18, children and teens need an average of about 9.5 hours of sleep per day, but healthy sleep times can range from 8 to 13 hours depending on age. Generally, children and teens need less sleep as they grow older.
  • Adults: Adults need between seven and nine hours of sleep each night.
  • Elderly People: As people age, their sleep may be shorter and involve waking up more often during the night. Despite these challenges, people over 65 still need seven to eight hours of sleep each night.

Many people understand the amount of sleep needed for optimal health, but still aren't getting enough sleep. Whether it’s because of longer hours at work or the temptation of social media and electronics, 30% of working American adults (15) receive less than six hours of sleep each night.

Optimizing Sleep

Understanding the science of sleep can help you recognize the importance of quality rest and take control of your sleep hygiene. Here are science-based tips for optimizing your sleep:

  • Allow Enough Time for Sleep: Getting enough sleep is necessary to reduce the body’s sleep drive and feel refreshed during the day. Many people won't sleep enough unless they intentionally carve out time for sleep. Find out how much sleep you need and make getting it a regular habit, even on the weekends.
  • Reduce Light Exposure Close to Bedtime: Light interferes with the body’s natural circadian rhythms and can make it more difficult to fall asleep. To reduce light exposure, turn off electronics at least an hour before bed and, if you work the night shift, be sure to use an eye mask or blackout curtains to eliminate light while sleeping.
  • Find Ways to Relax: Stress invokes the fight-or-flight response (16), which causes heart rate and blood pressure to increase. Being energized in this way is the opposite of what the body needs when falling asleep. Reduce stress at bedtime by finding ways to relax, like stretching, meditating, or reading a book.
  • Talk to a Doctor: Sleep is so crucial for health that anyone having issues sleeping should speak with their doctor. Doctors, sleep specialists, and counselors can address sleep issues and help you identify problems, so you can obtain quality rest.

There’s nothing quite like waking up refreshed after a good night’s rest. Your body and mind will thank you for making small changes to improve your sleep.


Related Reading:

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