Medically Reviewed by Samuel Mackenzie, MD, PhD
We do it every night, and over the course of our life we will spend approximately a third of our time doing it: sleep. But what is it? Doctors and scientists are really just beginning to understand all the important ways that sleep affects our health and well-being — and all of the reasons we do it.
According to Rafael Pelayo, MD, clinical professor of psychiatry and behavioral sciences and a sleep specialist at the Stanford Sleep Medicine Center in Redwood City, California, “Sleep is a natural, restorative, physiological process characterized by a perceptual disengagement [meaning you tune out from whatever’s going on around you], and must be rapidly reversible.”
Sleep experts at Harvard
Medical School’s Division of Sleep Medicine use a similar definition for
sleep. They say it can be characterized as: (1)
- A period of being less active
- A function of the body typically associated with a lying down posture and closed eyes
- A process whereby you’re less responsive to external stimuli
- A state of consciousness that’s easy to get out of (unlike other states of consciousness, such as hibernation or coma)
- Being associated with certain brain wave activity patterns and certain physiological changes, including a drop in blood pressure and body temperature
Regardless of the words used to describe it, the bottom line is that we need sleep to function, Dr. Pelayo says. It’s a critical process that allows the body to function and stay healthy — and it’s especially important for the brain.
“The entire body takes advantage of sleep,” Pelayo explains. For example, the kidneys slow down production of urine and digestion slows in the gut. (2) “But sleep is really how the brain gets reset for the next day. Sleep restores the brain.”
That means not getting enough sleep or good quality sleep will damage many systems of the body and over time can contribute to risk of chronic disease and health problems, but the most immediate consequences of not sleeping that you’ll notice are those that affect your mind and thinking.
Why Our Bodies Need Sleep — Tonight, and for Good Health Later On
We intuitively know we need sleep. When you don’t get a good night’s sleep, you’ll likely feel dozy, you won’t quite be able to think as clearly as usual, and you might be moody and irritable. That’s because one of the key functions of sleep is to restore the brain.
Why the Brain Needs Sleep
“Sleep is something the brain needs,” Pelayo explains. Our brains run on electricity, which means the chemical energy the brain uses to function has waste products (called metabolites) that need to get cleaned out. That’s what happens during sleep, Pelayo says. The brain flushes out those waste products in the brain and replenishes the energy the brain uses throughout the day (called adenosine-triphosphate, or ATP). (3
You likely won’t be measuring the ATP levels of your brain on a daily basis, but they do affect our functioning in big ways. If you don’t get a good night’s sleep and those chemical process don’t happen, the next day you’ll likely notice:
- It’s tougher to concentrate
- It’s harder to remember things (4)
- You’re moody and irritable (5)
- Your judgment might be skewed (6)
- You have less patience
- You’re more likely to make rash decisions or have a tough time making decisions
- You’re more emotional than usual
- Your hand-eye coordination is a little bit off
There’s also emerging evidence that over time, chronically not getting enough sleep could be linked to the buildup of certain proteins in the brain that are linked to problems like Alzheimer’s disease and other neurological problems. (7)
Why the Body Needs Sleep
Of course, it’s not just our minds that need sleep. Other systems of the body don’t work quite right when they’re too tired, either. Immediately after getting a poor night’s sleep you might notice you’re hungrier and tend to crave and eat more (8), and people are also at higher risk of catching a cold or flu. (9) Researchers think that’s because sleep deprivation has been shown to mess with how our immune systems function. (10)
Over time, chronically not sleeping well has also been shown to be linked to higher risk of chronic problems, including: (11)
- Type 2 diabetes
- Heart disease and hypertension
- Depression, anxiety, and other mood disorders
- Poor immune function
- Earlier death
Circadian Rhythm, Sleep Stages, and Sleep Cycles: Everything You Need to Know About What Happens When You Sleep
You may not remember everything that happens each night when you’re asleep, but if you’re doing it right, there’s a lot going on in your brain and your body, Pelayo says. “There are differences between sleep and awake for every single body system, but nothing as dramatic as the changes of consciousness during sleep, the brain function,” he explains.
The Different Stages of Sleep
During sleep the brain cycles, repeatedly, through different stages of sleep: (12)
Stage 1 Non-REM (Rapid Eye Movement) Sleep The first stage is when you’re actually falling asleep — stage 1 non-REM. Your heartbeat, breathing, and eye movement start to slow down and your muscles relax. Your brain waves are also slowing down and it’s still very easy to be awoken during this preliminary stage of sleep. (3)
Stage 2 Non-REM Sleep The second stage is when heart rates drops and body temperature falls even more. Eye movement stops completely and brain activity slows way down, other than brief bursts of activity.
Stage 3 Non-REM Sleep Next comes deep sleep. This is the stage of sleep that is heavy and restorative. Your heartbeat and breathing slow down the greatest during this stage of sleep and it is most difficult to be woken up.
REM Sleep Finally comes REM sleep, when your eyes begin to dart quickly back and forth from side to side (even though your eyelids are still closed). Brain activity speeds way up, closer to the amount of activity that happens when you’re awake. And this is the stage of sleep when most of your dreaming happens. Your breathing gets quicker and irregular during REM sleep. Heart rate and blood pressure start to go back up nearer to the speed they function at when you’re awake, though the muscles of your arms and legs become temporarily paralyzed during REM sleep. Sleep experts suspect this paralysis is a mechanism our bodies developed to help protect us from injury or other harm that might otherwise ensue if we were able to “act out” our dreams. (14)
Each cycle of sleep (which consists of all of the stages) usually takes about 90 minutes. And most people tend to spend more time during each cycle in deeper sleep earlier in the night — and more time in REM sleep later on. Each stage of sleep is important and both deep sleep and REM sleep play critical functions in terms of the learning and memory consolidation processes that happen during sleep. (15)
What Drives Sleep
Two internal systems control when we sleep and when we’re awake. First there’s the sleep-wake homeostatic drive. The longer we’re awake, the more our bodies crave sleep — and the longer we’re asleep, the more the body wants to wake up. The homeostatic sleep drive affects how deeply we sleep, too. For instance, if you were to stay awake for 24 or 36 hours instead of the typical amount of time you spend awake during a day, such as 16 or 17 hours, sleep-wake homeostasis is the mechanism that drives you to sleep longer and deeper.
Then there’s our circadian rhythm, our body’s biological clock, which is what helps sync our body functions with environmental cues. These internal clocks are what drive us to feel sleepy at night and more awake in the morning (even, for instance, if you slept poorly the previous night, or even pulled an all-nighter). They’re regulated by hormones, such as the stress hormone cortisol and the sleep hormone melatonin, which get secreted by the brain to send these wake and sleep signals to the body.
“They’re two complementary systems in the brain,” Pelayo says. And when there’s a discrepancy between the homeostatic drive to sleep and the signal to sleep that comes from the circadian system, problems like jet lag and other disordered sleep occurs.
“This is why people who wake up at different times every day may feel tired a lot,” Pelayo says. “The brain doesn’t know how to predict when they should be awake. It’s like being constantly jet-lagged.”
The more sleep researchers learn about these two systems that control sleep, the more it is clear why not only getting enough hours of sleep, but also having good sleep habits (such as going to sleep and waking up at the same time each day) is important.
How Much Sleep You Actually Need
How much sleep you actually need each night varies somewhat for each of us depending on our age (younger people typically need more sleep than adults) and our genes (some people are naturally shorter sleepers than others). But typically the sleep target for adults is between seven and nine hours each night, according to guidelines from the National Sleep Foundation. (16)
That recommendation, along with additional recommended sleep times for younger children, adolescents, and older adults, is based on the amount of sleep associated with the best health outcomes in a number of areas, including things like mood, learning, accidents, hypertension, heart disease, diabetes, and pain.
But Pelayo says don’t get too concerned about banking a specific number of hours of sleep each night. “The issue is waking up refreshed,” he says. “You should never wake up tired. If you do wake up feeling tired, something is wrong.”
Waking up sleepy could be an indicator that the quality of your sleep is poor. Maybe you’re spending too much time in light sleep, and not getting enough restorative deep sleep, for example, Pelayo says. If that’s the case, you should ask your doctor about getting checked for a sleep disorder, or see a sleep medicine specialist.
Common Sleep Disorders
Everyone should be able to get a good night’s sleep on a regular basis, Pelayo says. And if you’re not, it’s important to be aware of the several sleep disorders that might be interfering with your rest.
Here are definitions of some of the more common sleep disorders and how to recognize you may have one: (17)
Insomnia Insomnia is characterized by having difficulty falling asleep or staying asleep. Cases can be short term, such as those due to a stressful event, like a job change or jet lag; or long term, meaning the sleep trouble lasts for three months or longer, which is known as chronic insomnia. (18)
Obstructive Sleep Apnea (OSA) Obstructive sleep apnea, sometimes also referred to as just “sleep apnea,” is a disorder where someone’s airway becomes partially or completely blocked during sleep, which repeatedly wakes that person up during sleep and stops them from getting the deep, restorative sleep they need. People who are obese, have a small jaw or a large overbite, and use alcohol before bed are all at a higher risk of having sleep apnea.
Snoring and waking up not feeling rested, particularly after spending a full night asleep, are signs you might have sleep apnea and should get checked out by your doctor. Left untreated, sleep apnea can cause big problems, including high blood pressure, heart disease, memory problems, and higher accident risk.
Narcolepsy Narcolepsy is a disorder of the central nervous system that causes the brain to not be able to properly regulate cycles of sleep and being awake. (19) People with the disorder can experience the sudden, sometimes uncontrollable, need to fall asleep throughout the day, as well as trouble staying asleep at night.
Restless Legs Syndrome (RLS) RLS is a disorder that causes uncomfortable sensations in the legs and an irresistible urge to move them. (20) Symptoms are most likely to occur when you’re sitting, resting, inactive for a while, or sleeping. The condition is categorized as a neurological sensory disorder because the symptoms come from the brain — though it is also classified as a sleep disorder. It can cause exhaustion and daytime sleepiness that affect mood, concentration, learning, and relationships.
Parasomnias A parasomnia is term used to refer to a number of disorders associated with abnormal behaviors that happen during sleep. Parasomnias include sleepwalking, sleep-related eating disorder, sleep terrors, bedwetting, sexsomnia, and others. In some cases, improving sleep habits can help treat parasomnias and in other cases treatment from a sleep medicine doctor may be needed. You should definitely seek treatment if abnormal behavior associated with sleep is causing harm to yourself or others, or if the behavior is frequent or escalating. (21)
None of these problems should be left unaddressed, Pelayo says. If you suspect you may have one of these conditions it’s important to get it checked out and treated.
How to Actually Sleep Better Tonight
There’s no one foolproof formula for getting a good night’s sleep, but there are several steps you can take that have been associated with better sleep overall if you’re struggling to clock the recommended number of hours of sleep you know you need — or if you wake up feeling less rested than you want to be.
It’s important to check with your doctor or a sleep medicine doctor if you think you do have a more serious problem, or of another medical condition is interfering with your sleep.
But trying these fixes first is a good place to start:
Stick to a consistent sleep-wake schedule. Aim to go to bed at the same time each night and wake up at the same time in the morning, including on the weekends — and try not to vary it more than an hour or so. The times that you regularly go to bed and wake up are the signals you give your body’s natural clock, and when they’re consistent, that clock helps you wake up and fall asleep. If those signals are out of whack, your body clock gets thrown off and you experience the same drowsiness associated with jet lag. You also may struggle to fall asleep at night or wake up when your alarm rings.
Watch caffeine intake. Be especially careful with this later in the afternoon. Pelayo suggests avoiding caffeine within six hours of when you want to sleep.
Exercise regularly. Research shows that regular exercise (at least 150 minutes of activity per week) is associated with better sleep, (22) though it’s worth noting you should try to avoid intense exercise too close to bedtime, as it may make it tougher for some people to fall asleep. That’s because a workout sends signals to the body that tend to wake you up, such as your heart rate and body temperature increasing. (23)
Avoid bright lights and bright screens right before bed. Blue light — the kind that comes from fluorescent bulbs, LEDs, and computer and cellphone screens — has been shown to actually send the same signals to the brain as sunlight, and block production of the hormone melatonin that tells the brain to go to sleep. (24)
If you can’t sleep, don’t linger in bed. This means at night if you’re having trouble falling asleep for 20 minutes or longer, get out of bed and do something to make you tired, such as reading or some gentle stretching. Staying in bed makes your body associated in-bed time as awake time, and it will actually be harder to fall asleep.
Don’t linger in bed in the morning either, and don’t hit snooze. It can be tempting to wake up slowly, but that drowsy sleep (after you’ve initially woken up) is fragmented, light sleep. If you did get a poor night’s sleep, your best remedy is getting up, going about your day, and hitting your pillow at bedtime that evening, at which point your sleep drive will be strong and you’re more likely to actually reap the benefit of the deep restorative sleep you need.