Why Do We Sleep?

Not too many things beat settling into bed after a long day, but there’s more to snoozing than its feel-good benefits. So, why do we sleep?

Sleep is essential for feeling, and functioning at, your best. It affects everything from your memory to your mood to your immune system. Scientists have been studying sleep for decades and have come to one major conclusion: Good sleep is as important to your health as food and water.

The Importance of Sleep

Sleep is complex and, as such, all its functions have yet to be discovered. One thing we know for sure is that you need sleep to rest and recharge. But there are some other benefits of sleep that are more closely linked to your overall health and have less to do with feeling more energized the next day.

Energy Conservation

One theory is that we sleep to conserve energy. Because metabolic functions, like breathing and heart rate, slow down during sleep, we use fewer calories throughout the night. According to research, sleeping eight hours per night translates to a 35% energy savingsVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source.

This theory is based on the fact that back in the day, humans needed to spend the day hunting to find food (using a lot of energy), and often they had to go without a meal. Conserving energy at night was necessary for survival.

Brain Health

Brain health is another major function of sleep. Sleeping is your brain’s chance to restore, recharge and remove toxic byproducts that have built up during the dayVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source. This nocturnal “cleansing” can help improve learning and memory recall, reduce mental fatigue, and even help regulate your metabolism. A good night’s sleep helps you think more clearly, makes you more alert, and improves your attention span.

Sleep affects brain plasticity too. Getting the right amount of sleep improves your ability to perform tasks and learn new ones.

Mood and Emotional Regulation

Sleep is also important for regulating your emotionsVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source—when you’re sleep-deprived, you’re more likely to feel irritable and short-tempered. Poor sleep is also linked to higher stress levels overallVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source, and it can negatively impact the way you’re able to handle stress. In other words, when you’re well-rested, things tend to roll off your shoulders a little easier than when you haven’t gotten quite enough hours of sleep.

Immune Health

Before jumping into how sleep supports immunity, it’s helpful to understand the two main parts of your immune system. Your innate immune system is your “general” immunity. It responds quickly to bacteria and viruses to try to prevent them from entering the body. Anything that gets through your innate immune system has to deal with your adaptive immune system, which is more specific. Your adaptive immune system can identify germs and pathogens and sends out a targeted defense to destroy themVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source.

Sleep is important for both innate and adaptive immunity. It influences the hypothalamus-pituitary-adrenal (HPA) axis and the sympathetic nervous system (SNS), both of which play a roleVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source in regulating adaptive and innate immune responses.

When you sleep, levels of cortisol, epinephrine, and norepinephrine go down, while growth hormone, prolactin, and melatonin increase. However, when you don’t get enough sleep, it activates the HPA axis and the SNS, which can skew the action of these hormones and neurotransmitters.

In other words, levels of cortisol, epinephrine, and norepinephrine go up, while growth hormone, prolactin, and melatonin drop. This can cause a cascade of effects that suppresses the expression of your immune system.

Sleep also triggers an increase in specific immune cells, like interleukin-2 (IL2) and natural killer cells. When you sleep, the numbers of these protective cells go up to prepare you for the next day.

The Science of Sleep

While sleep seems simpleVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source, but it’s a pretty complicated process that involves several different hormones, as well as properly functioning “internal clocks.” There are some aspects of it that still aren’t completely understood by researchers, but there are two major playersVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source here: circadian rhythm and sleep/wake homeostasis.

Circadian rhythm: Your circadian rhythmVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source is a 24-hour biological cycle that controls everything in your body, including your sleep/wake cycle. This rhythm is governed by a central clock located in your brain, with your biological clock being highly sensitive to light.

Sleep/wake homeostasis: While circadian rhythm is important, sleep/wake homeostasis plays a major role in how alert or tired you feel throughout the day. Sleep/wake homeostasis the interaction between the circadian rhythms that make you feel alert and the sleep=-wake drive that regulates how tired or awake you feel. The longer you’re up, the more your sleep drive increases, but during the day it’s counteracted by the circadian rhythm of alertness. When you sleep at night, your sleep drive decreases.

REM Sleep vs Non-REM Sleep

There are two main types of sleep: rapid eye movement (REM) and non-rapid eye movement (NREM). Once you actually fall asleep, your body goes through four different stages of sleep (assuming you aren’t tossing and turning in the middle of the night)—three NREM stages and one REM stage.

While you might not think much is happening, each stage of sleepVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source is important. Your brain is still active as you’re snoozing, and you’re also forming memories.

The Four Stages of Sleep

• Stage 1 (N1): This is the light sleep stage when you start to drift off. You may feel like you’re somewhere in between consciousness, slightly aware but not fully present. During this stage, your brain waves start to slow down, as do your breathing rate, heart rate, and eye movements. Your muscles also relax as your body transitions from wakefulness to sleep. This stage lasts from one to five minutes and makes up about 5% of your total sleep time.
• Stage 2 (N2): In stage 2, everything continues to slow down and your body temperature drops. However, this stage is characterized by short bursts of brain activity, called sleep spindles and K-complexesVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source. This brain activity serves two main purposes: It helps keep you asleep so you’re not woken up easily by external stimuli and it plays a role in forming memories. This stage makes up about 45% of your total sleep time.
• Stage 3 (N3): Stage 3 sleep is another non-rem sleep stage that’s also called slow-wave sleep (SWS). This is the most refreshing and deepest stage of sleep—it’s very difficult to wake someone up when they’re in stage 3. This is also the stage when you reap all the aforementioned benefits of sleep. You spend about 25% of your total sleep time in stage 3.
• REM sleep: REM sleep is often confused with deep sleep, but it’s actually the most active stage, not the deepest. The first cycle of REM sleep happens around 90 minutes after you first fall asleep. Breathing, heart rate, and blood pressure increase and your brain waves and eye movements get faster. This is also where most dreaming occurs. The duration of this stage differs throughout the night, with the first cycle lasting about 10 minutes and the last closer to an hour. REM sleep makes up 25% of your total sleep time.

In one night, you’ll typically go through around four to five sleep cycles. Each cycle takes 90 to 110 minutes to move through the sleep stages. At the beginning of the night, you spend more time in non-REM sleep, but the longer you sleep, the more time you spend in REM sleep.

Memory consolidation—or the process of converting short-term memories to long-term ones—requires both NREM and REM sleep.

The Effects of Sleep Deprivation

You need to sleep. If you don’t get enough sleep or you get low-quality sleep—meaning you spend the night tossing and turning and don’t successfully go through all of the sleep stages—the sleep deprivation can leave you feeling tired and groggy. But there’s more to it than that.

Sleep deprivation can make it difficult to focus and may negatively impact your memory. Sleep deprivation can alsoVerified Source Harvard HealthBlog run by Harvard Medical School offering in-depth guides to better health and articles on medical breakthroughs.View source reduce motivation, impair reaction times, and lead to poor decision-making, while simultaneously lowering your immunity and making you more susceptible to illness.

Over time, a chronic lack of sleep may also contribute to high blood pressure, increase your risk of heart attack or stroke, make you more likelyVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source to develop insulin resistance and/or diabetes, and contribute to weight gain and obesityVerified Source National Library of Medicine (NIH)World’s largest medical library, making biomedical data and information more accessible.View source.

Research has also connected long-term sleep deprivation with dementia. In one study, researchers found that participants who slept fewer than five hours a night were twice as likely to develop dementia. In another study, researchers found that 50-, 60-, and 70-year old participants who slept six hours or less every night were 30% more likely to develop dementia than those who slept seven hours.

How Much Sleep Do You Need?

Now that you know why we sleep, the next question is “how much sleep do you need?” The answer depends on your age, according to the CDCVerified Source Centers for Disease Control and Prevention (CDC)The United States’ health protection agency that defends against dangers to health and safety.View source.

Adults require significantly less sleep than babies and kids, who need to sleep for more than half the day to feel their best. See also our guides on baby sleep cycles and how much sleep do kids really need.

While getting the recommended hours of sleep is important, you also want to make sure the sleep you do get is quality sleep. Lying in bed tossing and turning all night will do nothing for you. Setting a sleep schedule, keeping your bedroom cool, and using breathable bedding can help. However, if you need more ideas, check out the rest of our sleep tips.