How Does Sleep Work? Everything You Need To Know

Medically reviewed by
 Dr. Nayantara Santhi

Dr. Nayantara Santhi

Dr. Nayantara Santhi holds an academic position at Northumbria University. After completing her Ph.D. at Northeastern University (Boston, MA), she joined the Division of Sleep Medicine at Harvard Medical School as a post-doctoral fellow to research how sleep and circadian rhythmicity influence our cognitive functioning.

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Last Updated On October 4th, 2023
How Does Sleep Work? Everything You Need To Know

Key Takeaways

  • Sleep is Essential for Overall Health: Sleep plays a pivotal role in ensuring the proper functioning of the body. It is necessary for cellular repair, immune system maintenance, cognitive function, emotional regulation, and many other critical processes. Lack of sleep can lead to a wide range of health problems.
  • Sleep Consists of Different Stages: Sleep occurs in cycles consisting of different stages, including non-REM and REM sleep. Each stage serves various functions, such as memory consolidation, tissue repair, and emotional processing. Understanding these stages is important for comprehending the sleep process.
  • Sleep Quality Matters: Getting enough sleep is important, but the quality of sleep also matters. Factors such as sleep interruptions, sleep environment, and sleep disorders can affect the overall quality of your sleep. Monitoring your sleep patterns and addressing issues that disrupt your sleep can help improve your overall health and well-being.

We all need sleep but do we really know why? You may know that sleep is vital to your physical and mental health, and important for our bodies overall well-being. But what actually happens when you sleep? And why is it so essential?

Sleep plays a pivotal role in ensuring our bodies function properly, restoring and repairing cells overnight. And maintaining our circadian rhythm is a key part of getting a good night’s sleep. Here’s what actually goes on when you close your eyes at night — and what can happen as a result of not getting enough sleep.

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Why Do We Sleep?

There’s much about sleep that we don’t yet understand and that scientists and medical researchers have not yet been able to figure out. But what we do know is that sleep is a required function Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source  — it’s not something you can opt out of. It’s as vital as eating or even breathing and without it, we cannot survive.

We know that we feel more rejuvenated and refreshed when we get enough sleep. While many of the specific functions of sleep remain a mystery to scientists, we do know that we need to sleep in order to stay alive.

What Happens When You Sleep?

Right after you fall asleep, there are a few key changes Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source  in your body. Your brain activity changes, your body temperature lowers, and your heart rate slows. Even your breathing slows. In fact, your body expends less energy overall while you’re sleeping.

So what happens during this time? There are four sleep stages your body cycles through during the night. In a given night, you’ll cycle through each of these stages 4 to 6 times and each cycle (which includes all four stages) lasts for an average of 90 minutes. Each stage is marked non-REM or REM, which refers to rapid-eye movement.

“Traditionally, dreaming has been considered synonymous with rapid eye-movement (REM) sleep, which is characterized by wake-like, high-frequency brain electrical activity, notes Dr. Nayantara Santhi. “But research now shows that dreams also occur in NREM sleep. In both NREM and REM sleep, reports of dream experience are associated with frequency changes in brain electrical activity in specific brain areas.”

Here’s what happens during each stage.


This is not technically a stage of sleep, but is instead categorized as the stage where your body is not sleeping and your eyes are open. This stage is categorized by beta waves, which are higher-frequency waves that are seen when you’re alert or active. Alpha waves are more commonly spotted when you’re resting and when you’re asleep.

Stage 1 – Non-REM

Also known as N1 or Non-REM sleep stage one, this is the lightest stage of sleep and the first one you enter when falling asleep. This stage only lasts approximately one to five minutes and your breathing rate may actually remain normal during it (or slow slightly). It’s very easy to wake you in this stage of sleep.

This stage accounts for 5% of your sleep time.

Stage 2 – Non-REM

This next stage is also non-REM and is slightly deeper than stage one. Both your heart rate and body temperature drop and sleep spindles, which are marked by large bursts of neurons firing throughout your brain, occur. Sleep spindles are often linked with memory construction and organization.

If you suffer from bruxism — grinding your teeth at night — it occurs during this stage.

This stage typically lasts for 25 minutes during the first cycle, but lengthens during each cycle, ultimately accounting for 45% of your sleep time..

Stage 3 – Non-REM

This stage of deep sleep prepares you to enter REM sleep and is actually the deepest level of sleep. It’s the hardest stage to wake you from and, if awakened, can take you up to an hour to experience normal levels of processing.

During this stage, the body repairs your tissues and cells, strengthens your immune system, and even grows your bones and muscles. Those who suffer from sleepwalking or bedwetting can experience those symptoms during this stage.

As you grow older, your experience less of stage 3 non-REM sleep and more of stage 2 non-REM sleep. For most adults, stage 3 will take up roughly 25% of your total sleep time.

Stage 4 – REM Sleep

Many people have heard of REM sleep and mistakenly think it’s the deepest form of sleep — but not only does stage 3 take that title, but REM sleep is actually not very restful at all. It’s closely linked to dreaming and during this stage your muscles actually become limp, except for your eyes and breathing muscles. In fact, the irregular way your eye muscles move earned this stage its name. Your breathing also becomes more erratic.

Your brain is very active during REM sleep. In fact, your metabolism also increases by as much as 20% during this stage. In the morning, most people tend to wake up on their own during REM sleep.

Like stage 2 non-REM sleep, this stage of sleep also increases with each cycle. During the first cycle it lasts for about ten minutes, but by the last cycle it can last for about an hour. It typically occurs 90 minutes into your first sleep cycle.

Why Is Sleep So Important?

While we don’t fully know how sleep helps your body, there are some benefits we know sleep provides — particularly during stage 3 non-REM sleep.

Cellular Repair

During the night, your body actually repairs and strengthens damaged cells during stage 3 non-REM sleep. In fact, studies actually found that when you do not get enough sleep, the lack of sleep can actually lead to cell damage. Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source

Improved Cognitive Ability

It also appears that sleep plays a crucial role in your cognitive functioning. Not only does sleep help you organize and form memories, but it’s also tied to better overall cognitive functioning. Although we don’t fully understand how this works, studies focusing on sleep deprivation have found cognitive ability declines Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source  when sleep suffers.

Energy Conservation

We know you conserve energy while you sleep, but why does this happen? One theory suggests Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source  that energy conservation is the main role of sleep. Supporters of this theory note that primitive humans would have been less able to hunt for food at night, and thus, picking this time of day to sleep and conserve energy for the next day became imperative.

Emotional Regulation

Like many of the processes that happen during sleep, much of what we know is uncovered due to the negative side effects we experience when we do not get enough sleep. We know that sleep deprivation can lead to a dysfunction in regulating emotions.

Thus, it has been posited that sleep also can improve your ability Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source to regulate your emotions while awake. In fact, dreaming, which we know very little about, may also exist as a means to help us process our emotions and store memories.

Stronger Immune System

As mentioned above, stage 3 non-REM sleep is when your body works to strengthen your immune system. Your immune system is regulated by hormones which are produced while you sleep. As a direct result, poor sleep and circadian rhythm disruptions can lead to a weakened immune system. Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source

Helps Maintain Your Metabolism

Have you ever wondered when trying a new weight loss program or exercise regime why sleep is so heavily emphasized? It’s because it plays a vital role in maintaining your metabolic rate, which can have an impact on how your body burns calories.

While you sleep, your body repairs and maintains its metabolic processes, which can help keep your metabolism working normally. But, disrupted sleep and not enough hours of sleep can disturb your metabolism, Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source  leading to hormonal imbalance and weight issues.

Hormone Production

Your growth hormone levels actually increase at night while you sleep, particularly in the first two stages of sleep. Hormones can play key roles in maintaining your health — and when depleted can lead to serious health issues. Those suffering from sleeping disorders Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source  may notice their hormones are also out of whack.

Protects Against Insulin Resistance

Sleep has also been linked to insulin resistance, putting those with sleeping disorders at higher risks of obesity, Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source  insulin resistance, and diabetes. While healthy sleep won’t cure diabetes or insulin issues, maintaining healthy sleep may prevent these diseases — or at least lower your risks.

Supports Cardiovascular Health

According to the American Heart Association, sleep can also impact your heart health. This can be directly — those who receive less sleep are at a higher risk of heart attack or other cardiovascular diseases — or indirectly. Not getting enough sleep increases your risk of weight gain, eating an unhealthy diet, and overall inflammation in your body — all which increase your chances of heart issues.

How Does Your Body Regulate Sleep?

Now that you understand what happens during sleep, the stages of it, and how it can impact your health, let’s dig into how your body regulates when you need to sleep and when you should wake.

Your body maintains sleep through two processes: sleep/wake homeostasis and your circadian biological clock.

Your sleep/wake homeostasis is fairly basic. The longer you’re awake, the greater your need for sleep. If this process was fully in charge of your sleep cycle, you’d be tired at the end of the day and ready for sleep, and most alert after waking up.

But, your circadian biological clock interferes. Have you ever felt a 2 pm slump? Thank your biological clock for that. This process experiences highs and lows of sleepiness throughout the day.

The daytime increase in the circadian arousal signal counteracts the wake-dependent increase in homeostatic sleep pressure allowing us to stay alert through the waking day. It is the opposing changes in the circadian and homeostatic sleep-wake propensity that keeps us alert during the day and asleep at night.

How Much Sleep Do You Really Need?

So, we need sleep, but how much of it? While the exact number of hours of sleep Verified Source Centers for Disease Control and Prevention (CDC) The United States’ health protection agency that defends against dangers to health and safety. View source you need may vary, adults should get seven hours of sleep every night. Any less can lead to negative health side effects.

More than nine hours of sleep, Verified Source National Library of Medicine (NIH) World’s largest medical library, making biomedical data and information more accessible. View source however, may be beneficial for teenagers and those suffering from lack of sleep, along with those with illnesses (particularly seniors). Younger children need more sleep, with infants requiring up to 15 hours of sleep and children under twelve needing 9 to 12 hours.

Your doctor can run tests and perform a sleep study if you’re worried you aren’t getting enough sleep.

How Can You Tell If You’re Getting Enough Sleep?

A good way to tell if you’re getting enough sleep is to keep track of when you go to bed and wake up. If it’s under seven hours, you’re likely not getting enough sleep. But, if you wake frequently in the night — even for a few minutes — you may also be suffering from sleep deprivation.

When you interrupt your sleep cycle, it starts over again. Frequently waking up Verified Source Cleveland Clinic Ranked #2 hospital by U.S. News & World Report and one of the largest academic medical centers in America. The Cleveland Clinic serves patients from all over the world. View source throughout the night could lead to you not getting enough deep sleep or REM sleep.

If you feel rested after sleeping, you’re probably getting enough sleep. But if you feel groggy and irritable — especially for hours after waking — and you struggle to get up with your alarm clock, you may be sleep deprived.

What Happens if You Don’t Get Enough Sleep?

Not getting enough sleep can lead to sleeping disorders like insomnia, which is when it’s difficult to fall asleep, or even sleep anxiety, where you’re afraid you won’t be able to fall asleep, and thus become too anxious to rest. It can also lead to sleep apnea and chronic insomnia.

There are mental and emotional implications, too. Your memory will be less clear, you may experience brain fog, and your cognitive abilities are lessened. You may find you’re irritated and unable to regulate your emotions, as well. It also puts you at a higher risk for depression and anxiety, as well as other mental disorders. In fact, there’s a strong link between mental health and your sleeping habits.

Physically, your body may feel weaker. And there are even worse potential effects when it comes to sleep’s impact on physical health. Consistently not getting enough sleep can put you at a higher risk for obesity, heart disease, diabetes, and other serious diseases. It can also worsen symptoms of other diseases or disorders you’re suffering from.

Not getting enough sleep is a serious issue. It wreaks havoc on your body and can have major consequences. Talk to your doctor if you’re worried you aren’t getting the right amount of sleep.

Frequently Asked Questions

Why is sleep important for our bodies?

Sleep plays a crucial role in maintaining good physical and mental health. During sleep, the body repairs and regenerates tissues, strengthens the immune system, and consolidates memories. Lack of sleep has been linked to numerous health problems, including obesity, diabetes, heart disease, and depression.

How does the brain control sleep and wake cycles?

The brain’s sleep-wake cycle is controlled by two systems: the circadian system and the sleep homeostatic system. The circadian system regulates the timing of sleep and wakefulness and is influenced by light exposure, while the sleep homeostatic system regulates the need for sleep based on how long we have been awake. The interaction between these two systems helps determine when we feel alert and when we feel drowsy.

What factors affect sleep quality?

Several factors can affect the quality of our sleep, both internal and external. Such factors include stress, diet, exercise, and our sleep environment. Avoiding caffeine before bedtime, establishing a consistent sleep schedule, and creating a relaxing sleep environment can all help improve sleep quality.

How does lack of sleep affect our physical and mental health?

Lack of sleep can have a significant impact on our physical and mental health. Chronic sleep deprivation has been linked to a range of health problems, including weight gain, diabetes, heart disease, and depression. In addition, lack of sleep can impair cognitive function, memory, and reaction time, making it difficult to concentrate and perform tasks.

How can I fall asleep quickly?

Establishing a consistent sleep routine, avoiding caffeine before bedtime, and creating a relaxing sleep environment can all help promote faster sleep onset. Other tips include avoiding electronic devices before bedtime, practicing relaxation techniques such as deep breathing exercises, and engaging in regular exercise for better sleep. It is also important to maintain a consistent sleep schedule, even on weekends.


Sleep plays a crucial role in keeping your body healthy and functioning properly. It impacts your mental, emotional, and physical health and failure to get enough sleep can lead to serious health risks.

While we know there are links between sleep and important bodily functions, there’s much we don’t know about this process. Ultimately, we know we must sleep, but we don’t fully understand all of the reasons why.

About the author

Courtney Johnston is a seasoned freelance writer and editor with over 10 years of experience in publishing digital content. Her areas of expertise include personal finance, small business, and health and wellness. With her work published in reputable outlets such as The Chicago Tribune, MSN, AOL, The Motley Fool, Benzinga, The Balance, Best Reviews, and The Culture Trip, Courtney brings a wealth of knowledge and a strong editorial background to her writing.

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