What Happens When You Sleep?


Sleep is integral to your physical, emotional, and mental health. A good night's rest allows the body to recover and recharge, leaving you feeling refreshed and energetic when you wake up. Physiological processes that occur during sleep are tied to critical bodily functions such as cellular restoration, energy conservation, weight maintenance, insulin production, and heart health.

By the same token, inadequate or low-quality sleep can make you more susceptible to diseases, infections, and mental health disorders, and also cause different impairments during the day. How much sleep you need each night depends largely on your age. Good sleep hygiene can help ensure you get the rest you need in order to stay healthy and happy.

How Does Sleep Work?

Every person has an internal timekeeping system known informally as the "circadian clock," which is located in the hypothalamus near the front of the brain. The circadian clock is programmed to reset, or "entrain," every 24 hours. This 24-hour cycle, the circadian rhythm, is guided by natural light and plays a major role in hormone production, as well as mood, appetite and digestion, body temperature, and other bodily functions.

This clock consists of roughly 20,000 nuclei clustered together to form a structure called the suprachiasmatic nucleus (SCN). During the day, the retinas in your eyes perceive natural sunlight and transmit signals through a nerve tract that leads directly to the SCN. These signals inform the brain whether it is day or night.

In the evening as natural light begins to disappear, the pineal gland in your brain will produce melatonin, a natural hormone that induces feelings of relaxation and sleepiness. When you wake up in the morning and your eyes perceive natural light, the body will produce another hormone, cortisol, that promotes alertness and wakefulness. The brain stem also communicates with the hypothalamus to produce GABA, a hormone that decreases arousals and helps the body wind down.

In addition to circadian rhythm, your sleep is also regulated by a process called sleep-wake homeostasis. Also known as your sleep drive, this mechanism regulates feelings of tiredness and wakefulness. For every hour you're awake, your sleep drive will become stronger, and these feelings will culminate right before you go to bed.

Your circadian rhythm and sleep-wake homeostasis do not exist in a vacuum. Circadian rhythm disorders can cause you to feel tired and alert at times of the day that do not align with natural light cycles. Examples range from mild conditions such as jet lag to more serious conditions such as advanced or delayed sleep-wake phase disorder, irregular sleep-wake rhythm disorder, and shift work disorder. Factors that can affect or alter your sleep-wake homeostasis include light exposure, diet, stress, medical conditions, and your sleep environment.

What Are the Stages of Sleep?

The term sleep architecture refers to the physical structure of your sleep cycle. The sleep cycle of a healthy adult consists of four distinct stages. The first three stages are considered non-rapid eye movement (NREM) sleep, and the last stage is considered rapid eye movement (REM) sleep.

  • NREM 1: The first stage signifies the transition between wakefulness and sleep. NREM consists of light sleep characterized by a gradual reduction to your heartbeat, breathing rate, eye movements, and brain wave activity. The muscles will also begin to relax, though they may twitch – movements known as hypnic jerks or sleep starts. This stage usually lasts several minutes.
  • NREM 2: The second stage also consists of light sleep, though your heartbeat, breathing rate, eye movements, and brain wave activity will drop to lower levels than during NREM 1. Your body temperature will also decrease significantly and eye movements will completely cease. NREM 2 is the longest of the four sleep stages.
  • NREM 3: This stage marks the beginning of slow-wave, or deep, sleep. Heartbeat, breathing rates, and brain wave activity will decrease to their lowest possible levels and the muscles will completely relax. NREM 3 is a longer stage when you first fall asleep but it will gradually shorten throughout the night. Historically, sleep experts believed the sleep cycle also included a second slow-wave stage known as NREM 4, but current nomenclature combines the two slow-wave stages into a single stage (NREM 3).
  • REM: The final stage of your sleep cycle will first occur about an hour and a half after you nod off. As the name suggests, your eyes will move erratically beneath your eyelids, and your brain waves will become more active. Breathing rates also increase, and your heart rate and blood pressure elevate to levels that are closer to those during wakeful periods. Dreaming primarily takes place during this stage, as well, and your muscles will be temporarily paralyzed; this bodily mechanism prevents you from physically responding to dreams.

Overall, each sleep cycle usually lasts between 90 and 120 minutes. The duration of each stage largely depends on your age, since people spend less time in REM sleep as they get older.

How Much Sleep Does a Person Need?

The average adult should receive at least seven hours of sleep for every 24-hour cycle. Sleeping less than seven hours can leave you more vulnerable to different diseases and medical conditions, and also affect your ability to concentrate and increase your risk of errors and accidents. That said, the sufficient amount of sleep for a given individual depends on their age.

Sleeper Group Age Range Recommended Daily Sleep
Infant 4-12 months 12-16 hours (including naps)
Toddler 1-2 years 11-14 hours (including naps)
Preschool 3-5 years 10-13 hours (including naps)
School age 6-12 years 9-12 hours
Teen 13-18 years 8-10 hours
Adult 18+ 7-9 hours

Source: Centers for Disease Control


Insufficient sleep creates a problem known as sleep debt. Let's say you are an adult 18 years or older who receives six hours of sleep every night. Just one week of inadequate rest produces seven hours of sleep debt. Napping can provide a quick pick-me-up, but it does not deliver the same restorative functions of nightly sleep. As a result, chronic sleep deprivation can lead to serious health problems down the road.

Why Is Sleep Important?

The physiological changes that occur during sleep help to regulate different bodily functions, including:

Brain function: Each distinct sleep stage helps to restore cells in the brain and optimize its daytime performance. The brain will essentially reorganize itself and create new pathways to help you learn new information and consolidate memories. At the same time, it flushes out "toxic" byproducts that can negatively impact brain health and performance.

  • Heart health: Your blood pressure decreases while you sleep, which makes it easier for your cardiovascular system to function. If you don't get enough sleep, this means your blood pressure will remain at higher levels for longer periods of the 24-hour sleep-wake cycle.
  • Immunohealth: The immune system is essentially your body's defense shield against diseases and infections. While the immune system doesn't shut down during sleep, certain aspects are more active than others.
  • Insulin production: Insulin is a hormone that regulates how much glucose, or sugar, is in your blood. Your glucose levels increase following meals, so your body produces more insulin. When you sleep, your glucose and insulin levels decrease.
  • Energy conservation: Many experts believe sleep is vital to health because it helps us conserve energy. Your metabolism operates at lower levels while you're asleep, which decreases the amount of energy you need. One study estimates eight hours of sleep can conserve up to 35% of the energy you require to function during the day.
  • Weight maintenance: Your body produces hormones in response to appetite and digestion. These include ghrelin, which makes you feel hungry, and leptin, which tells you you're full. Lack of sleep can elevate your ghrelin levels while decreasing your leptin levels. As a result, you may eat more during the day than you otherwise might after a good night's sleep.
  • Growth and development: Deep sleep promotes production of hormones that regulate growth in children and adolescents. These hormones also increase muscle mass and play a role in cell repair and restoration. This is one of the reasons why sufficient sleep is particularly important for children and teens.

What Happens to Your Body When You Don't Sleep?

Sleep deprivation often leads to tiredness during the day. You may wake up feeling less refreshed and alert than you might after seven to nine hours of uninterrupted sleep, and this can affect how you perform in different professional and social settings. Other immediate side effects of inadequate sleep include anger and irritability, impulsivity and poor decision-making, and trouble concentrating.

Chronic lack of sleep can be very detrimental to your long-term health. Complications that may arise include:

Heart disease and stroke: Sleep allows your blood pressure levels to temporarily  decrease during certain stages of the sleep cycle. Since blood pressure drops during sleep, less sleep means a smaller percentage of your 24-hour circadian cycle is spent in this low blood pressure zone. High blood pressure can be a risk factor for heart disease and stroke.

  • Obesity: Inadequate sleep interferes with production of your ghrelin and leptin hormones that regulate feelings of hunger and satiation, respectively. This may cause you to eat more during the day, which can lead to unhealthy weight gain and obesity.
  • Type 2 diabetes: Sleep plays a role in insulin production and glucose levels. Not getting enough rest can elevate your blood sugar to unhealthy levels and put you at higher risk of Type 2 diabetes. Additionally, obesity is considered a major risk factor for this disease.
  • Poor immunohealth: Without enough time to restore itself, your immune system's defense barriers can weaken due to inadequate sleep. This makes it harder for your body to stave off diseases and infections, putting you at a higher risk of getting sick.
  • Stunted growth and development: The hormones that regulate growth and development, build muscle mass, and repair cells in children and adolescents are triggered during deep sleep. Those who don't receive enough sleep may not grow and develop properly.
  • Accidents and injuries: Lack of sleep decreases your ability to concentrate and reduces your reaction time. This puts you at higher risk of hurting yourself, especially at work or while driving. Roughly 100,000 car accidents – and 1,500 deaths – occur each year due in part to driver sleepiness.
  • Impaired performance: People who don't get enough sleep are less likely to succeed in school and tend to commit more errors at work. Even minor sleep debt can contribute to these impairments and negatively affect your overall wellbeing and cognitive performance.

Tips for Getting Enough Sleep Each Night

Practicing sleep hygiene is crucial to receiving an adequate amount of rest on a nightly basis. Key aspects of sleep hygiene include:

Follow a consistent sleep schedule: Go to bed and wake up at the same times, even on the weekends or when you're on vacation.

  • Maintain a relaxing sleep area: A quiet bedroom with limited exposure to outside light will promote healthier sleep than a louder, brighter setting. You should also maintain a comfortable bedroom temperature for sleeping since your body temperature will decrease during sleep. Most experts agree 65 degrees Fahrenheit (18.3 degrees Celsius) is the ideal temperature for most people, but anywhere from 60 to 67 degrees should be comfortable enough for sleep.
  • Exercise regularly: Some physical activity during the day can help you expend energy and feel tired before going to sleep. However, you should avoid exercise during the late afternoon and evening hours leading up to bedtime.
  • Avoid naps after the mid-afternoon: Napping can provide short-term restoration that affects how sleepy you feel in the evening.
  • Spend some time in the sunlight: Not getting outside enough during the day can affect your circadian rhythm and make it harder for your body to regulate its sleep-wake cycle. A stroll during your lunch hour or in the afternoon helps maintain a healthy circadian clock.
  • Watch your intakes: You should avoid stimulants like caffeine and nicotine late in the day, as these can offset the effects of hormones like melatonin that make you feel tired. Also, don't drink alcohol before bed. While alcohol has sedative properties that help you fall asleep more quickly, you may experience fragmented sleep as your liver enzymes break down the alcohol during the night.
  • Avoid heavy nighttime meals: Foods that are heavy in fat or sugar can affect how much you sleep at night, as well as your overall sleep quality. Light snacks are recommended instead, especially soporific foods that contain GABA, melatonin, and other sleep-inducing components. Examples of good foods to eat before bed include whole grains, walnuts, cherries, and milk.
  • Don't use electronics before bedtime: Cell phones, televisions, computers, and other electronic devices emit artificial blue light that can trick your brain into thinking it's daytime and affect your circadian rhythm. Some of these devices are equipped with filters that minimize your exposure to this light, but for best results, try meditating or taking a hot bath before bed instead.

Chronic sleep deprivation should be taken seriously. If you aren't getting enough sleep on a regular basis, consult with your doctor or another credentialed physician about ways to address this issue and improve your sleep allotment.


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