What Happens During Sleep


Written by Dr. Michael Breus

Our Editorial Process

Table of Contents

Sleep is a period of rest during which your body gets a break from daytime stress and activity. Although the body is relaxed during sleep, the mind is hard at work. In fact, what happens during sleep affects almost every bodily system.

Sleep is just as important to your health as food and water. What happens during sleep affects how you think, feel, and function during the day. Good sleep leaves you feeling your best and comes with a range of benefits — from a better mood to enhanced learning and memory.

Considering that you spend around one-third of your life asleep, it’s understandable that you might be curious about what happens during your nightly rest. Understanding how sleep works can empower you to improve your sleep health and get the full benefits of a good night’s rest.

Key Takeaways


  • Sleep is essential for overall health and well-being.
  • Sleep consists of four different stages, including NREM and REM sleep.
  • A full sleep cycle promotes physical restoration, cognitive function, memory consolidation, and emotional well-being.
  • Recommended sleep duration varies by age, with adults needing 7-9 hours of sleep per night.

What Are Sleep Cycles?

A sleep cycle is a period of time in which the body cycles through two different types of sleep. During a sleep cycle, the brain alternates between rapid eye movement (REM) and non-rapid eye movement (NREM) sleep. People typically go through five or six sleep cycles every night.

Sleep cycles are usually shorter early in the night, with the first sleep cycle lasting only 70 to 100 minutes. As the night progresses, sleep cycles grow longer and can last around 90 to 120 minutes. The length and pattern of sleep cycles also vary based on a person’s age, mood, recent sleep habits, and medication use, including caffeine and alcohol.

REM Sleep vs. NREM Sleep

During non-rapid eye movement (NREM) sleep, the body progresses from a relatively light sleep to a deeper sleep from which it’s more difficult to be awakened. As the body moves into a deeper and more restful state, breathing and heart rate slow, blood pressure drops, and the eyes do not move. More than 75% of sleep time is spent in NREM sleep.

Rapid eye movement (REM) sleep makes up the remainder of time spent asleep. During REM sleep, breathing and heart rate become quicker, and blood pressure increases to levels near those found during waking hours. Muscles throughout the body become temporarily paralyzed during REM to prevent movement, while the eyes dart rapidly back and forth. Periods of REM sleep tend to become longer though the night.

While researchers are still learning about the function of REM sleep, REM sleep is often associated with dreaming. Interestingly, dreams can occur during both NREM and REM sleep, but those that happen during REM are often the most vivid.

What Are Sleep Stages?

The two types of sleep are further categorized into four sleep stages, each associated with distinct patterns in brain waves, eye movements, and other changes in the body. NREM sleep is composed of three stages and REM sleep represents the final sleep stage.

Sleep Stage
Sleep Category

Stage 1


During this lightest stage of sleep, heartbeat and breathing start to slow as the muscles and eyes relax. Brain waves shift from waking patterns to those seen during sleep.

1 to 7 minutes

Stage 2


As the body transitions into a slightly deeper form of light sleep, it continues to relax, and eye movements stop. Brain waves are slow, with brief upticks in activity.

10 to 25 minutes

Stage 3


This period of deep sleep, often referred to as slow wave sleep, is linked to feeling well-rested the following day. As blood pressure, heart rate, and breathing reach their lowest points, brain activity continues to slow.

20 to 40 minutes

Stage 4


During REM, heart rate and breathing speed up as eye movements, blood pressure, and brain activity increase. The majority of dreams occur during REM.

10 to 60 minutes

While the reason we sleep is not fully understood, each sleep stage is associated with characteristic changes in the body that support the health and function of systems like the nervous system, cardiovascular system, and immune system.

Stage 1 is very light sleep that occurs when a sleeper is dozing off and the body begins to shift from being awake to being asleep. Heartbeat and breathing start to slow, muscles relax, and brain activity begins to change. Because it only lasts a few minutes during each sleep cycle, people who wake up during this stage may not realize that they were asleep.

Stage 2 is another light period of sleep, occurring as the body begins to transition to deep sleep. This stage is characterized by continued relaxation of the muscles, slowing of the heartbeat and breathing, and a drop in body temperature. As brain waves continue to slow, there are occasional short bursts in brain activity. In a typical night, more time is spent in this stage than any other stage of sleep.

Stage 3 is a time of deep sleep in which muscles are relaxed while heartbeat and breathing are the slowest of the night. As brain activity continues to slow, sleepers are most resistant to waking up from noises and other stimuli during this stage. Stage 3 is particularly important for feeling refreshed after waking up. When people are woken during this stage, they often feel foggy and disoriented for up to an hour afterward.

Stage 4 is rapid eye movement sleep (REM), named for the quick pace at which the eyes dart back and forth behind closed eyelids during this stage. Heart rate, blood pressure, and brain activity increase to rates similar to those seen while awake. Breathing becomes irregular during this stage, and the muscles of the arms and legs are temporarily paralyzed to prevent movement. Periods of REM sleep are shorter during the first several sleep cycles and become longer closer to awakening.

When we’re in stage 3 and 4 sleep, the glymphatic system pulls the waste out of the brain and cleans out the system.
Dr. Michael Breus

What Affects Your Sleep Cycles and Stages?

How an individual moves through sleep cycles is called their sleep architecture. Maintaining good sleep architecture is vital for physical and mental health. Sleep architecture varies based on a multitude of individual factors, including a person’s age, body mass index (BMI), and sex. A person’s sleep architecture also shifts naturally during the course of their life.

For example, the amount of time a person spends in REM sleep is linked to important periods of brain development. Instead of transitioning through the sleep stages beginning with stage 1, infants enter sleep through REM sleep, and spend half or more of their sleep time in this sleep stage. Time spent in REM sleep falls to around 30% by six months of age and to a quarter or less of sleep time by 5 years old.

More than one-third of people in the United States don’t get the recommended amount of sleep for their age group. Sleep deprivation can dramatically affect a person’s sleep architecture. Insufficient sleep may lead to more time spent in stage 3 sleep and experiencing REM rebound. In REM rebound, the body compensates for missed sleep by spending more time in REM sleep.

Other factors that affect sleep architecture include:

  • Noise: Whether loud or relatively quiet, sounds in a person’s sleep environment can cause more nighttime awakenings and lead to more time spent in lighter stage 1 sleep.
  • Alcohol: Drinking alcohol may significantly affect sleep architecture, suppressing REM sleep and leading to more time spent in lighter sleep stages.
  • Stress: Research has shown that daytime stress can affect sleep in a variety of ways, such as by impacting how long it takes a person to reach the REM sleep stage and making it harder to remember dreams. Stress can also lead to REM rebound.
  • Medications: Many prescription and over-the-counter medications can change a person’s sleep cycles. Several medications decrease time spent in deep sleep, like benzodiazepines used to treat sleepwalking, insomnia, and anxiety.
  • Sleep Disorders: Numerous sleep disorders affect sleep architecture. For example, in people with sleep apnea, disrupted breathing leads to less time spent in deeper stage 3 and REM sleep. Another sleep disorder, called narcolepsy, is characterized by skipping the early sleep stages and going from wakefulness directly into REM sleep.

Benefits of Sleep

Although scientists are still studying the many ways that sleep benefits the body, research has demonstrated that sleep supports physical health, mental well-being, and a high quality of life.

Benefits of Sleep for Your Body

The benefits of sleep for your body can’t be overstated. From fewer seasonal colds to healthier skin, the benefits of sleep include:

  • Muscle Recovery: Sleep allows the body to repair tissues, muscles, and bones. Much of muscle recovery is believed to occur during stage 3 sleep.
  • Improved Immunity: Sleep boosts the immune system and helps the body fight illness. People who don’t get enough sleep are at a greater risk of colds and other infections.
  • Weight Maintenance: The body regulates hormones during sleep that affect hunger and appetite. Scientists have found that people who do not obtain enough sleep are at higher risk for weight gain.
  • Skin Repair: Research suggests that sleep affects the skin’s ability to recover from damage. One study found that people who regularly fall short on sleep tend to have increased signs of aging and are less satisfied with their appearance.

Benefits of Sleep for Your Brain

Sleep is also important for thinking clearly and feeling good. The brain-boosting benefits of sleep include:

  • Memory Formation: Getting enough sleep is important for making long-term memories. Sleep stage 2, which involves a type of brain activity called “sleep spindles,” is believed to be particularly important for memory consolidation.
  • Optimal Learning: During sleep, the brain forms new pathways that help with learning and retaining new information. Sleep also improves problem-solving skills, attention, and creativity.
  • Happier Mood: Sleep helps people control their emotions. In contrast, a lack of sleep makes it harder to enjoy positive experiences and increases negative reactions.

How to Tell If You Are Sleeping Well

According to experts, healthy sleep involves three factors: sleep quantity, sleep quality, and sleep schedule consistency.

The amount of sleep you need depends on your age, health, and if you have recently lost any sleep. For most adults, getting 7 to 9 hours of sleep each night is needed for optimal functioning. Children and adolescents typically need more sleep as they grow and develop.

Sleep quality is based on your ability to stay asleep during the night, whether you feel refreshed after waking up, and how long it takes you to fall asleep. Maintaining a consistent sleep schedule helps the body relax and prepare for falling asleep.

If you’re concerned about how well you are sleeping, talk to a doctor or sleep specialist. Doctors can use a variety of tools to evaluate sleep architecture and diagnose sleep disorders, ultimately helping you get the sleep you need to feel well.

About The Author

Dr. Michael Breus

Clinical Psychologist, Sleep Medicine Expert

Michael Breus, Ph.D is a Diplomate of the American Board of Sleep Medicine and a Fellow of The American Academy of Sleep Medicine and one of only 168 psychologists to pass the Sleep Medical Specialty Board without going to medical school. He holds a BA in Psychology from Skidmore College, and PhD in Clinical Psychology from The University of Georgia. Dr. Breus has been in private practice as a sleep doctor for nearly 25 years. Dr. Breus is a sought after lecturer and his knowledge is shared daily in major national media worldwide including Today, Dr. Oz, Oprah, and for fourteen years as the sleep expert on WebMD. Dr. Breus is also the bestselling author of The Power of When, The Sleep Doctor’s Diet Plan, Good Night!, and Energize!

  • POSITION: Combination Sleeper
  • TEMPERATURE: Hot Sleeper

Ask the Sleep Doctor

Have questions about sleep? Submit them here! We use your questions to help us decide topics for articles, videos, and newsletters. We try to answer as many questions as possible. You can also send us an emailPlease note, we cannot provide specific medical advice, and always recommend you contact your doctor for any medical matters.