/ How Sleep Works / What Happens When You Sleep?
Eric Suni Staff Writer
Eric Suni
Staff Writer
Eric Suni has over a decade of experience as a science writer and was previously an information specialist for the National Cancer Institute.
Want to read more about all our experts in the field?
Ealena Callender OBGYN
Ealena Callender
OBGYN
Dr. Callender is a board-certified obstetrician-gynecologist who has been working in women’s health for over a decade.
Want to read more about all our experts in the field?
Fact-Checked
Sleep Foundation
Fact-Checking: Our Process
The Sleep Foundation editorial team is dedicated to providing content that meets the highest standards for accuracy and objectivity. Our editors and medical experts rigorously evaluate every article and guide to ensure the information is factual, up-to-date, and free of bias.
The Sleep Foundation fact-checking guidelines are as follows:
- We only cite reputable sources when researching our guides and articles. These include peer-reviewed journals, government reports, academic and medical associations, and interviews with credentialed medical experts and practitioners.
- All scientific data and information must be backed up by at least one reputable source. Each guide and article includes a comprehensive bibliography with full citations and links to the original sources.
- Some guides and articles feature links to other relevant Sleep Foundation pages. These internal links are intended to improve ease of navigation across the site, and are never used as original sources for scientific data or information.
- A member of our medical expert team provides a final review of the content and sources cited for every guide, article, and product review concerning medical- and health-related topics. Inaccurate or unverifiable information will be removed prior to publication.
- Plagiarism is never tolerated. Writers and editors caught stealing content or improperly citing sources are immediately terminated, and we will work to rectify the situation with the original publisher(s)
- Although Sleep Foundation maintains affiliate partnerships with brands and e-commerce portals, these relationships never have any bearing on our product reviews or recommendations. Read our full Advertising Disclosure for more information.
Table of Contents
Table of Contents
- How Does Sleep Change During the Night?
- What Happens to Your Brain and Body During Sleep?
- What Happens When You Have Problems Sleeping?
When you sleep, your body undergoes a series of changes that enable the rest that is vital to your overall health. Sleep allows the brain and body to slow down and engage in processes of recovery, promoting better physical and mental performance the next day and over the long-term.
What happens when you don’t sleep is that these fundamental processes are short-circuited, affecting thinking, concentration, energy levels, and mood. As a result, getting the sleep you need — seven to nine hours for adults and even more for children and teens — is crucial.
What happens during sleep, including how distinct stages of sleep unfold, demonstrates the complexity of sleep and its importance for our well-being.
Is Your Troubled Sleep a Health Risk?
A variety of issues can cause problems sleeping. Answer three questions to understand if it’s a concern you should worry about.
Please select all options
How Does Sleep Change During the Night?
During a normal sleep period, you progress through four to five sleep cycles. Each sleep cycle is made up of four individual sleep stages.
The four stages of sleep are further broken down into two categories: rapid eye movement (REM) and non-REM sleep. These categories are important because what happens during REM sleep is dramatically different from what happens during non-REM stages.
The first three stages of sleep are composed of non-REM activity. Stage 1 is short, representing the act of dozing off and transitioning into sleep. In Stage 2 the body and mind slow down as you settle into sleep. It’s easiest to be awoken during these first two stages.
In Stage 3, also known as deep sleep, the body is in recovery mode, slowing down even further. At the same time, overall brain activity slows and shows a tell-tale pattern of pulses of activity Trusted Source National Library of Medicine, Biotech InformationThe National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.View Source that are believed to help prevent unwanted awakenings.
The fourth stage is REM sleep. During REM periods, brain activity shoots back up to levels similar to when you’re awake – which explains why REM is associated with the most intense dreams. While breathing and heart rate increase during REM sleep, most muscles are paralyzed, which keeps us from acting out those vivid dreams.
Each sleep cycle takes between 70 and 120 minutes Trusted Source Division of Sleep Medicine at Harvard Medical SchoolA production of WGBH Educational Foundation and the Harvard Medical School Division of Sleep Medicine.View Source . In the first sleep cycles of the night, more time is spent in non-REM sleep. The majority of REM sleep happens during the second half of the night. The progression of sleep stages and cycles in one sleep period is known as sleep architecture.
What Happens to Your Brain and Body During Sleep?
Virtually every part of the body experiences notable changes during sleep. Upon falling asleep, thousands of neurons in the brain switch from waking to sleeping state Trusted Source National Library of Medicine, Biotech InformationThe National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.View Source , sending signals throughout the body.
While the biological role of sleep still isn’t fully understood, research demonstrates that it reinforces the cardiovascular and immune systems and helps regulate metabolism. What happens during sleep can be seen in notable changes in core bodily processes.
Breathing
Breathing slows during non-REM sleep with respiration reaching its lowest rates during deep sleep stage three. Breathing ramps up and may become irregular during REM sleep.
Heart Rate
As with breathing, heart rate begins to slow during Stage 1 and reaches its slowest pace during Stage 3. On the other hand, during REM sleep, the pulse quickens to nearly the same rate as when awake.
Muscle Tone
Muscles gradually relax during each stage of non-REM sleep, and the body’s total energy expenditure drops Trusted Source National Library of Medicine, Biotech InformationThe National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.View Source . During the REM stage, most muscles are paralyzed in a condition known as atonia. This keeps the legs and arms from flailing in response to dream content. Respiratory and eye muscles stay active, though, and the darting of the eyes behind closed eyelids is the inspiration for the name rapid eye movement sleep.
Brain Activity
When measured during sleep, brain waves show clear patterns associated with each sleep stage. In the early parts of non-REM sleep, brain waves slow down considerably; however, in Stage 2 and Stage 3, there are numerous quick bursts ofbrain activity.
In REM sleep, brain activity accelerates, showing markedly different types of brain waves. Heightened brain activity is why REM sleep is known as the stage most associated with vivid dreaming.
REM sleep is thought to enable critical cognitive abilities Trusted Source Division of Sleep Medicine at Harvard Medical SchoolA production of WGBH Educational Foundation and the Harvard Medical School Division of Sleep Medicine.View Source , including memory consolidation, but non-REM sleep, even with reduced brain activity, is also believed to play a role in facilitating proper brain function while awake.
Dreaming
Dreaming is most prevalent and intense during REM sleep, but it can occur during any sleep stage Trusted Source National Library of Medicine, Biotech InformationThe National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.View Source . That said, dreams that happen during non-REM and REM sleep tend to show different patterns Trusted Source National Library of Medicine, Biotech InformationThe National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.View Source with REM dreams often being more fanciful, immersive, or bizarre.
Hormone Levels
Sleep and the body’s internal clock, or circadian rhythm, play an important role in regulating the production of numerous hormones Trusted Source National Library of Medicine, Biotech InformationThe National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.View Source including:
- Melatonin, which helps promote sleep
- Growth hormone, which supports bone and muscle development as well as metabolism
- Cortisol, which is part of the body’s stress response system
- Leptin and ghrelin, which help control appetite
Hormone levels fluctuate during different sleep stages, and quality of sleep may also affect daytime hormone production.
What Happens When You Have Problems Sleeping?
When you have sleeping problems, you may not get the restorative benefits that come from what normally happens during sleep. The specific effects depend on the type of sleeping problem and its cause.
What Happens if You Have Insomnia?
People with insomnia have a hard time falling asleep or staying asleep for as long as they want to, which means that they get insufficient total sleep. As a result, they may not progress through enough sleep cycles to get proper rest, leading to daytime sleepiness as well as negative effects on mood and thinking.
Sleep deprivation, which often occurs with insomnia, can throw off the balance of sleep architecture. For example, after going without enough sleep, people often experience a REM sleep rebound Trusted Source National Center for Biotechnology InformationThe National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.View Source , spending a disproportionate amount of time in REM sleep. This can cause too much brain activity, which in turn can leave you feeling irritable and may worsen mental health issues like anxiety and depression.
What Happens During Sleep if You Have a Sleep Disorder?
Sleep disorders can negatively affect what happens when you sleep. For example, restless leg syndrome or disrupted breathing from sleep apnea can cause frequent awakenings that interrupt the normal sleep cycle, reducing restorative sleep. Circadian rhythm sleep-wake disorders can lead to insufficient sleep or abnormal sleep architecture.
What Happens When You Sleep Too Much?
Hypersomnia is a condition marked by sleeping too much. People with hypersomnia often experience excessive daytime sleepiness and may find it hard to stay awake when they need to. Studies indicate that hypersomnia is associated with changes in sleep architecture Trusted Source National Library of Medicine, Biotech InformationThe National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information.View Source , such as a reduction in deep sleep and an increase in NREM sleep, which may affect overall sleep quality.
Written By
Eric Suni,Staff Writer
Eric Suni has over a decade of experience as a science writer and was previously an information specialist for the National Cancer Institute.
Medically Reviewed by
Ealena Callender,OBGYNMD
Dr. Callender is a board-certified obstetrician-gynecologist who has been working in women’s health for over a decade.
References
10 Sources
Schönauer, M., & Pöhlchen, D. (2018). Sleep spindles. Current biology : CB, 28(19), R1129–R1130.
https://linkinghub.elsevier.com/retrieve/pii/S0960982218309345Division of Sleep Medicine at Harvard Medical School. (2007, December 18). Natural Patterns of Sleep., Retrieved October 16, 2020, from
http://healthysleep.med.harvard.edu/healthy/science/what/sleep-patterns-rem-nremSaper, C. B., Fuller, P. M., Pedersen, N. P., Lu, J., & Scammell, T. E. (2010). Sleep state switching. Neuron, 68(6), 1023–1042.
https://pubmed.ncbi.nlm.nih.gov/21172606/Jung, C. M., Melanson, E. L., Frydendall, E. J., Perreault, L., Eckel, R. H., & Wright, K. P. (2011). Energy expenditure during sleep, sleep deprivation and sleep following sleep deprivation in adult humans. The Journal of physiology, 589(Pt 1), 235–244.
https://pubmed.ncbi.nlm.nih.gov/21059762/Division of Sleep Medicine at Harvard Medical School. (2007, December 18). Sleep, Learning, and Memory., Retrieved October 16, 2020, from
http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/learning-memoryPagel, J. F. (2000). Nightmares and disorders of dreaming. American Family Physician, 61(7), 2037–2042, 2044.
https://pubmed.ncbi.nlm.nih.gov/10779247/Payne, J. D., & Nadel, L. (2004). Sleep, dreams, and memory consolidation: the role of the stress hormone cortisol. Learning & memory (Cold Spring Harbor, N.Y.), 11(6), 671–678.
https://pubmed.ncbi.nlm.nih.gov/15576884/Kim, T. W., Jeong, J. H., & Hong, S. C. (2015). The impact of sleep and circadian disturbance on hormones and metabolism. International journal of endocrinology, 2015, 591729.
https://pubmed.ncbi.nlm.nih.gov/25861266/Feriante J, Singh S. (2020, July 19). REM Rebound Effect. StatPearls Publishing., Retrieved from
https://www.ncbi.nlm.nih.gov/books/NBK560713/Plante D. T. (2018). Nocturnal sleep architecture in idiopathic hypersomnia: a systematic review and meta-analysis. Sleep medicine, 45, 17–24.
https://pubmed.ncbi.nlm.nih.gov/29680423/
Learn More About How Sleep Works
How to Become a Morning Person
ByDanielle Pacheco May 13, 2024
How Much Sleep Do You Need?
ByEric Suni May 13, 2024
How Memory and Sleep Are Connected
ByDanielle Pacheco May 9, 2024
What Causes Excessive Sleepiness?
ByDanielle Pacheco May 8, 2024
What Causes Restless Sleep?
ByEric Suni May 8, 2024
Biphasic Sleep: What It Is And How It Works
BySarah Shoen April 26, 2024
Polyphasic Sleep: Benefits and Risks
ByDanielle Pacheco April 26, 2024
Sleep Inertia: How to Combat Morning Grogginess
ByDanielle Pacheco April 11, 2024
REM Rebound: Causes and Effects
ByJay Summer April 11, 2024
REM Rebound: Causes and Effects
ByJay Summer April 11, 2024
Do Moon Phases Affect Your Sleep?
ByJay Summer April 5, 2024
Why Do We Need Sleep?
ByLucy Bryan April 5, 2024
Alpha Waves and Sleep
ByJay Summer April 1, 2024
How Age Affects Your Circadian Rhythm
ByDanielle Pacheco March 19, 2024
How Is Sleep Different For Men and Women?
ByLucy Bryan March 19, 2024
Circadian Rhythm
ByLucy Bryan March 15, 2024
Chronotypes: Definition, Types, & Effect on Sleep
ByDanielle Pacheco March 1, 2024
Sleep Drive and Your Body Clock
ByEric Suni March 1, 2024
8 Health Benefits of Sleep
ByJay Summer February 29, 2024
Daylight Saving Time: Everything You Need to Know
ByDanielle Pacheco February 27, 2024
How To Get a Good Night’s Sleep in a Hotel
ByDanielle Pacheco February 22, 2024
Does Napping Impact Your Sleep at Night?
ByDanielle Pacheco February 2, 2024
Does Daytime Tiredness Mean You Need More Sleep?
ByLucy Bryan February 2, 2024
Why Do I Wake Up at 3 am?
ByJay Summer January 19, 2024
Sleep Debt: The Hidden Cost of Insufficient Rest
ByRob Newsom January 16, 2024
Sleep Satisfaction and Energy Levels
ByDanielle Pacheco December 22, 2023
How Sleep Works: Understanding the Science of Sleep
ByEric Suni December 22, 2023
What Makes a Good Night's Sleep
ByDanielle Pacheco December 22, 2023
Sleep and Social Media
ByRob Newsom December 22, 2023
Orexins
ByRob Newsom December 22, 2023
Adenosine and Sleep: Understanding Your Sleep Drive
ByLucy Bryan December 14, 2023
Oversleeping
ByAustin Meadows December 8, 2023
Hypnagogic Hallucinations
ByJay Summer December 8, 2023
Hypnopompic Hallucinations
ByJay Summer December 8, 2023
What All-Nighters Do To Your Cognition
ByJay Summer December 1, 2023
Long Sleepers
ByEric Suni November 13, 2023
How to Wake Up Easier
ByJay Summer November 8, 2023
Sleep Spindles
ByJay Summer October 23, 2023
Does Your Oxygen Level Drop When You Sleep?
ByJay Summer October 18, 2023
100+ Sleep Statistics
ByEric Suni September 26, 2023
Short Sleepers
ByEric Suni June 2, 2023
How Electronics Affect Sleep
ByDanielle Pacheco June 2, 2023
Myths and Facts About Sleep
ByEric Suni June 1, 2023
What’s the Connection Between Race and Sleep Disorders?
ByEric Suni June 1, 2023
Sleep Latency
ByDanielle Pacheco January 18, 2023
Microsleep: What Is It, What Causes It, and Is It Safe?
ByJay Summer January 6, 2023
Light Sleeper: What It Means and What To Do About It
ByDanielle Pacheco January 3, 2023
Load More Articles