Why Do We Sleep?

Why Do We Sleep?

Sleep is essential for good health. In fact, we need sleep to survive — just like we need food and water. So, it’s no wonder we spend about one-third of our lives sleeping.

Many biological processes happen during sleep:

  • The brain stores new information and gets rid of toxic waste.
  • Nerve cells communicate and reorganize, which supports healthy brain function.
  • The body repairs cells, restores energy, and releases molecules like hormones and proteins.

These processes are critical for our overall health. Without them, our bodies can’t function correctly.

Let’s take a closer look at why we sleep, along with what happens if we don’t get enough.

Why do we sleep? 

A lot is still unknown about the purpose of sleep. However, it’s widely accepted that there isn’t just one explanation for why we need to sleep. It’s likely necessary for many biological reasons. 

To date, scientists have found that sleep helps the body in several ways. The most prominent theories and reasons are outlined below. 

Energy conservation

According to the energy conservation theory, we need sleep to conserve energy. Sleeping allows us to reduce our caloric needs by spending part of our time functioning at a lower metabolism. 

This concept is backed by the way our metabolic rate drops during sleep. Research suggests that 8 hours of sleep for human beings can produce a daily energy savings of 35 percent over complete wakefulness. 

The energy conservation theory of sleep suggests that a main purpose of sleep is to reduce a person’s energy use during times of the day and night, when it’s inconvenient and less efficient to hunt for food.

Cellular restoration 

Another theory, called the restorative theory, says the body needs sleep to restore itself. 

The idea is that sleep allows cells to repair and regrow. This is supported by many important processes that happen during sleep, including:

Brain function

The brain plasticity theory says sleep is required for brain function. Specifically, it allows your neurons, or nerve cells, to reorganize. 

When you sleep, your brain’s glymphatic (waste clearance) system clears out waste from the central nervous system. It removes toxic byproducts from your brain, which build up throughout the day. This allows your brain to work well when you wake up.

Research suggests that sleep contributes to memory function by converting short-term memories into long-term memories, as well as by erasing, or forgetting, unneeded information that might otherwise clutter the nervous system.

Sleep affects many aspects of brain function, including:

Emotional well-being

Similarly, sleep is necessary for emotional health. During sleep, brain activity increases in areas that regulate emotion, thereby supporting healthy brain function and emotional stability.

Areas of the brain in which sleep increases activity include:

One example of how sleep can help regulate emotion occurs in the amygdala. This part of the brain, located in the temporal lobe, is in charge of the fear response. It’s what controls your reaction when you face a perceived threat, like a stressful situation.

When you get enough sleep, the amygdala can respond in a more adaptive way. But if you’re sleep-deprived, the amygdala is more likely to overreact. 

Research shows that sleep and mental health are intertwined. On the one hand, sleep disturbances can contribute to the onset and progression of mental health issues, but on the other hand, mental health issues can also contribute to sleep disturbances.

Weight maintenance

Sleep affects your weight by controlling hunger hormones. These hormones include ghrelin, which increases appetite, and leptin, which increases the feeling of being full after eating.

During sleep, ghrelin decreases because you’re using less energy than when you’re awake.

Lack of sleep, however, elevates ghrelin and suppresses leptin. This imbalance makes you hungrier, which may increase the risk of eating more calories and gaining weight.

Recent research shows that chronic sleep deprivation, even as few as five consecutive nights of short sleep, may be associated with increased risk of: 

Proper insulin function

Insulin is a hormone that helps your cells use glucose, or sugar, for energy. But in insulin resistance, your cells don’t respond properly to insulin. This can lead to high blood glucose levels and, eventually, type 2 diabetes.

Sleep may protect against insulin resistance. It keeps your cells healthy so they can easily take up glucose.

The brain also uses less glucose during sleep, which helps the body regulate overall blood glucose.


A healthy and strong immune system depends on sleep. Research shows that sleep deprivation can inhibit the immune response and make the body susceptible to germs.

When you sleep, your body makes cytokines, which are proteins that fight infection and inflammation. It also produces certain antibodies and immune cells. Together, these molecules prevent sickness by destroying harmful germs.

That’s why sleep is so important when you’re sick or stressed. During these times, the body needs even more immune cells and proteins.

Heart health

While the exact causes aren’t clear, scientists think sleep supports heart health. This stems from the link between heart disease and poor sleep.

The Centers for Disease Control and Prevention (CDC) says the average adult needs 7 hours of sleep a night. Getting less than that on a regular basis can lead to health problems, many of which can hurt your heart health.

Lack of sleep is associated with risk factors for heart disease, including:

What happens when you sleep?

Your body cycles through four stages of sleep. This cycle occurs multiple times throughout the night for different lengths of time, varying from 70 to 120 minutes each. The stages generally repeat about four to give times during a 7- to 9-hour sleep period.

The pattern includes two major phases of sleep: non-rapid eye movement (non-REM) sleep and REM (rapid eye movement) sleep. The four stages of sleep include three stages of non-REM sleep and one stage of REM sleep.

As the names suggest, non-REM sleep features an absence of eye movements, whereas REM sleep, when dreaming occurs, is characterized by rapid eye movements.

The four stages of sleep are listed below.

Stage 1: Non-REM sleep

Stage 1 occurs when you first fall asleep. As your body enters light sleep, your brain waves, heart rate, and eye movements slow down.

This phase lasts for about 7 minutes.

Stage 2: Non-REM sleep

This stage involves the light sleep just before deep sleep.

Your body temperature decreases, your eye movements stop, and your heart rate and muscles continue to relax. Your brain waves briefly spike then slow down.

During a night of sleep, you spend the most time in stage 2.

Stage 3: Non-REM sleep

In stages 3 and 4, deep sleep begins. Your eyes and muscles don’t move, and your brain waves slow down even further. 

Deep sleep is restorative. Your body replenishes its energy and repairs cells, tissues, and muscles. You need this phase to feel awake and refreshed the next day.

Stage 4: REM sleep

This stage first happens about 90 minutes after you fall asleep. Your eyes move quickly from side to side during REM sleep.

In REM sleep, your brain waves and eye movements increase. Your heart rate and breathing also speed up. 

Dreaming often happens during REM sleep. Your brain also processes information during this stage, making it important for learning and memory. 

How much sleep do you need?

The recommended amount of sleep depends on your age. It also varies from person to person, but the CDC suggests the following durations based on age:

  • birth to 3 months: 14 to 17 hours 
  • 4 to 12 months: 12 to 16 hours per 24 hours, including naps 
  • 1 to 2 years: 11 to 14 hours per 24 hours, including naps
  • 3 to 5 years: 10 to 13 hours per 24 hours, including naps
  • 6 to 12 years: 9 to 12 hours
  • 13 to 18 years: 8 to 10 hours
  • 18 to 60 years: 7 or more hours
  • 61 to 64 years: 7 to 9 hours
  • 65 years and older: 7 to 8 hours

What happens if you don’t get enough sleep?

Without enough sleep, your body has a hard time functioning properly. Sleep deficiency is linked to chronic health problems affecting the heart, kidneys, blood, brain, and mental health. 

Lack of sleep is also associated with an increased risk of injury for both adults and children. Driver drowsiness, for example, can contribute to serious car accidents and even death. 

In older adults, poor sleep is associated with an increased risk of falls and broken bones.

Specific consequences of sleep deprivation can include:

Bottom line:

Sleep keeps us healthy and functioning well. It lets your body and brain repair, restore, and reenergize.

If you don’t get enough sleep, you might experience side effects like poor memory and focus, weakened immunity, and mood changes.

Most adults need 7 to 9 hours of sleep each night. If you’re having trouble sleeping, talk to your doctor or a sleep specialist. They can determine the underlying cause and help improve the quality of your sleep.

How long can we go without sleep?

How long can you go?

The longest recorded time without sleep is approximately 264 hours, or just over 11 consecutive days. Although it’s unclear exactly how long humans can survive without sleep, it isn’t long before the effects of sleep deprivation start to show. 

After only three or four nights without sleep, you can start to hallucinate. Prolonged sleep deprivation can lead to: 

Although dying from sleep deprivation is extremely rare, it can happen. 

Read on to find out how staying awake for a full 24 hours or more can affect your body, and how much sleep you actually need to function. 

What to expect after 24 hours without sleep

Missing 24 hours of sleep isn’t uncommon. You might miss a night of sleep to work, cram for a test, or take care of a sick child. While it might be unpleasant to stay up all night, it won’t have a significant impact on your overall health. 

Still, missing a night of sleep does affect you. Studies have compared 24-hour wakefulness to having a blood alcohol concentration of 0.10 percent. This is above the legal limit to drive in most states. 

Some effects of going 24 hours without sleep include: 

  • drowsiness
  • irritability
  • impaired decision-making
  • impaired judgement
  • altered perception
  • memory deficits
  • vision and hearing impairments
  • decreased hand-eye coordination
  • increased muscle tension
  • tremors
  • increased risk of accidents or near misses 

Symptoms of 24-hour sleep deprivation usually go away once you’ve had some shut-eye. 

What to expect after 36 hours without sleep

Staying awake for just 36 hours can have intense effects on your body. 

Your sleep-wake cycle helps regulate the release of certain hormones, including cortisol, insulin, and human growth hormone. As a result, going without sleep for an extended period of time can alter several bodily functions. 

This includes your: 

  • appetite
  • metabolism 
  • temperature 
  • mood 
  • stress level

Some effects of going 36 hours without sleep include: 

  • extreme fatigue
  • hormonal imbalances
  • decreased motivation
  • risky decisions
  • inflexible reasoning
  • decreased attention
  • speech impairments, such as poor word choice and intonation

What to expect after 48 hours without sleep

After two nights of missed sleep, most people have difficulty staying awake. They might experience periods of light sleep that can last up to 30 seconds. During these “microsleeps,” the brain is in a sleeplike state. Microsleeps happen involuntarily. After a microsleep, you might feel confused or disoriented. 

Staying awake for 48 hours also disrupts the immune system. Inflammatory markers, which help your body prevent and target illnesses, start to circulate at increased levels. Some research has shown that natural killer (NK) cell activity decreases with sleep deprivation. NK cells respond to immediate threats to your health, such as viruses or bacteria. 

What to expect after 72 hours without sleep

After 72 hours without sleep, most people experience an overwhelming urge to sleep. Many are unable to stay awake on their own. 

Going three days without sleep profoundly limits the ability to think, especially executive functions such as multitasking, remembering details, and paying attention. This level of sleep deprivation can make it difficult to see even simple tasks through to completion. 

Emotions are also affected. People who have undergone this level of sleep deprivation may be easily irritated. They may experience a depressed mood, anxiety, or paranoia. Research has also found that sleep deprivation makes it more difficult to process others’ emotions. In one study, participants with 30 hours of sleep deprivation had difficulty recognizing angry and happy facial expressions. 

Finally, several days of sleep deprivation can significantly alter perception. You might experience hallucinations, which occur when you see something that isn’t there. Illusions are also common. Illusions are a misinterpretation of something that’s real. An example is seeing a sign and thinking it’s a person. 

Can food and water intake have any effect on this?

Sleep deprivation can change both your appetite and the types of foods you crave. Studies suggest that sleep deprivation is associated with both an increased appetite and an increased desire for foods associated with weight gain. However, consuming empty calories can ultimately leave you more tired. 

Eating well may offset some of the effects of sleep deprivation, but only to an extent. Since your body is conserving energy, opt for lean, protein-rich foods, such as nuts and nut butters, cottage cheese, or tofu. Avoid fatty proteins, such as steak or cheese. These will make you sleepier. 

Dehydration can exacerbate the effects of sleep deprivation — such as grogginess and difficulty concentrating — so it’s also important to drink plenty of water.

What if sleep deprivation becomes chronic?

Chronic partial sleep deprivation is when you don’t get enough sleep on a regular basis. It’s different than pulling an all-nighter once in a while. It’s also more common than missing one or two nights of sleep in a row, as most people are likely to sleep for at least a few hours per night. 

The Centers for Disease Control and Prevention (CDC) report that 35 percent of American adults don’t get enough sleep per night. Chronic partial sleep deprivation is associated with both short-term health risks and long-term complications.

Not getting enough sleep over a short period, such as a week, may cause: 

  • anxiety
  • unstable mood
  • drowsiness
  • forgetfulness
  • difficulty concentrating
  • difficulty staying alert
  • cognitive impairments
  • decreased performance at work or school
  • increased risk of illness or injury

In the long term, not getting enough sleep can reduce immune functioning and increase your risk of certain health conditions. These include: 

How much sleep do you really need?

The amount of sleep you need per night varies according to your age. In general, newborns and infants need more sleep, and adults need less sleep. 

The CDC have daily sleep recommendations based on age group: 


Daily sleep recommendations


14-17 hours


12-16 hours


11-14 hours

preschool-age children

10-13 hours

school-age children

9-12 hours


8-10 hours


7-9 hours

Gender may also play a role in how much sleep you need. Studies have found that women tend to sleep slightly longer than men, although the reasons for this are unclear.

Sleep quality is also important. If you’re concerned about how much sleep you’re getting, make an appointment with your doctor. 

Bottom line:

It isn’t clear how long humans can truly survive without sleep. But it is clear that extreme symptoms can begin in as little as 36 hours. This includes a reduced ability to think, poor decision-making, and speech impairment. 

Pulling an all-nighter once every couple of months likely won’t do any long-term damage. But if they’re happening more often — intentionally or not — talk to your doctor. 

If you’re staying awake out of necessity, your doctor may be able to offer advice on how to do so in the most health-conscious way. Otherwise, your doctor can get to the root of your symptoms and help you get your sleep schedule back on track. 

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