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 that 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 overall health. Without them, your body can’t function correctly.

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

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.

According to the energy conservation theory, we need sleep to save energy. This concept is backed by the way our metabolic rate drops during sleep.

It’s also said this happens because the body needs less energy at night, when it’s inconvenient to find food.

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:

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.

Sleep affects many aspects of brain function, including:

Similarly, sleep is necessary for emotional health. During sleep, brain activity increases in areas that regulate emotion, including the:

This change in activity supports proper brain function and emotional stability.

For example, the amygdala 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.

Sleep affects your weight by controlling hunger hormones. This includes ghrelin, which increases appetite, and leptin, which increases satiety.

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 weight gain.

Insulin is a hormone that helps your cells use glucose 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.

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.

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

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

Your body cycles through four stages of sleep. The pattern typically repeats every 90 minutes. This means the stages will repeat about 4 to 6 times during a 7- to 9- hour sleep period.

The pattern includes three phases of non-rapid eye movement (NREM) sleep and one phase of REM sleep.

The NREM sleep stages used to be divided into stages 1, 2, 3, and 4, followed by REM sleep. The National Sleep Foundation now classifies them as follows:

N1 non-REM sleep (formerly stage 1)

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.

N2 non-REM sleep (formerly stage 2)

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.

N3 non-REM sleep (formerly stages 3 and 4)

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.

REM sleep

This stage first happens about 90 minutes after you fall asleep. It can last for about an hour.

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.

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

  • Birth to 3 months: 14 to 17 hours
  • 4 to 11 months: 12 to 15 hours
  • 1 to 2 years: 11 to 14 hours
  • 3 to 5 years: 10 to 13 hours
  • 6 to 13 years: 9 to 11 hours
  • 14 to 17 years: 8 to 10 hours
  • 18 to 64 years: 7 to 9 hours
  • 65 years and older: 7 to 8 hours

Without enough sleep, your body has a hard time functioning properly.

Possible consequences of sleep deprivation include:

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 swings.

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