When falling asleep at night, it might feel like your brain and body are “shutting down” or switching off for a few hours. In fact, a variety of complicated things happen in your body when you’re asleep — both obvious and not-so-obvious. Yes, you’re dreaming (which you may or may not remember) and snoring lightly or… not so lightly. But you are also consolidating memories, repairing muscles and cells, and so much more.
While you sleep, your body moves through different sleep stages in two types of sleep: Non-REM sleep (NREM) and rapid eye movement sleep (REM). You spend roughly 75% of your sleeping time in NREM sleep, which is sometimes also called slow-wave sleep (SWS).
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The National Sleep Foundation describes the different sleep stages as follows:
- Stage 1 sleep is an extremely light stage, and it’s fairly easy to be woken up.
- During Stage 2 sleep, body temperature drops and breathing and heart rate become regular.
- Stage 3 sleep is described as the “deepest and most restorative sleep.” During this stage, blood pressure drops, muscles relax, breathing slows down, and hormones are released.
- Finally, you reach REM sleep, also known as Stage 4 sleep, which first happens about 90 minutes after falling asleep. This stage is characterized by your eyes moving back and forth — hence the name, rapid eye movement. During REM sleep, your breathing can speed up and become irregular, and dreaming can occur as your brain is extremely active. “Signals are sent to the brain’s cerebral cortex, which is responsible for learning, thinking, and organizing information,” the National Institutes of Health (NIH) explains. “Signals are also sent to the spinal cord to shut off movement, creating a temporary inability to move the muscles (‘paralysis’) in the arms and legs.”
- After a period of REM sleep, one full sleep cycle is complete and Stage 1 sleep starts again.
In each of these stages, different things happen in your brain and muscles. Plus, your hormones and nervous system are active throughout the night. This piece will cover what’s going on in various parts of your body while you are sleeping, from brain activity to muscle repair, the functions of different organs, and more.
As you can imagine, a lot is going on inside the brain during sleep — and not just when you are dreaming. According to the NIH, different sleep stages are associated with different brain wave activity. Your brain waves slow down through Stage 1 and Stage 2, reaching their slowest point in Stage 3 sleep. But when you reach REM sleep, your brain activity “becomes closer to that seen in wakefulness.” And some people do actually “act out” their REM dreams, physically moving their bodies around while they sleep.
“We do know that a small group of cells in the brain stem, called the subcoeruleus nucleus, controls REM sleep,” a Scientific American article explains. “When these cells become injured or diseased, people do not experience the muscle paralysis associated with REM sleep, which can lead to REM sleep behavior disorder—a serious condition in which the afflicted violently act out their dreams.”
Sleep is also linked to memory consolidation. Harvard Health’s Healthy Sleep website breaks down learning and memory into three steps: Acquisition (introducing new information to the brain), consolidation (making a memory stable) and recall (the ability to access the information or memory after it has been introduced and stabilized).
“Each of these steps is necessary for proper memory function,” the site explains. “Acquisition and recall occur only during wakefulness, but research suggests that memory consolidation takes place during sleep through the strengthening of the neural connections that form our memories. Although there is no consensus about how sleep makes this process possible, many researchers think that specific characteristics of brain waves during different stages of sleep are associated with the formation of particular types of memory.”
Plus, how well-rested you are can affect your acquisition and consolidation before learning, too. “We’ve learned that sleep before learning helps prepare your brain for the initial formation of memories,” Dr. Matthew Walker, a sleep scientist at the University of California, Berkeley, told the NIH’s News In Health. “And then, sleep after learning is essential to help save and cement that new information into the architecture of the brain, meaning that you’re less likely to forget it.”
Body Temperature Regulation
The brain also helps regulate body temperature during sleep, via a small portion of the brain called the hypothalamus. According to MedLine Plus, the hypothalamus helps regulate temperature along with appetite, sex drive, and the sleep-wake cycle. Your body temperature starts to lower in Stage 2 sleep, thanks to cues from your brain.
Research published in the journal Neuroscience Research in 2017 further explains the relationship between the hypothalamus and your sleep-wake cycle. The paper identifies two types of neurons in the hypothalamus, one type “a crucial component for maintenance of wakefulness” and the second “implicated in the regulation of sleep/wakefulness.”
And, of course, your brain is involved in dreaming. No one can say for sure exactly why we dream or what the purpose of dreaming is, but some research says that dreaming is linked to memory consolidation. According to LiveScience, analyzing brain waves can reveal whether or not someone is dreaming, as proven by research published in 2017. “The researchers found that, when the participants were dreaming, they showed a decrease in low-frequency brain waves, and an increase in high-frequency brain waves, in the posterior hot zone, compared with when they weren’t dreaming. They found this pattern regardless of whether the participants were dreaming during REM or non-REM sleep.”
A variety of different hormones are released to help you fall asleep and wake up. The National Sleep Foundation explains that the endocrine system — made up of glands around the body that secrete hormones into your blood where they can be transported — has a “complex response to sleep.” According to the site, hormones like growth hormone, prolactin, and luteinizing hormone are secreted at higher rates during sleep, while others, like thyroid-stimulating hormone and cortisol, are inhibited during sleep.
One major hormone involved with sleep is melatonin. The University of Rochester Medical Center website explains that melatonin is a hormone made by the pineal gland, and melatonin secretion is controlled by light exposure. Research published in the British Journal of Pharmacology in 2018 on the relationship between melatonin and the circadian body clock says that the “light‐inhibited production of melatonin conveys the message of darkness to the clock and induces night‐state physiological functions, for example, sleep/wake blood pressure and metabolism.”
The relationship between melatonin and light is the reasoning behind many common sleep hygiene recommendations. For example, have you ever been told to avoid looking at electronic screens that emit blue light for at least an hour before bed? That’s because that type of light can interfere with melatonin secretion, making it more difficult for you to fall asleep. Many devices now offer “night mode” to address this issue, but it’s still ideal to ditch your phone, tablet, or TV for a book or audiobook when you are winding down before bed.
Knowing about melatonin secretion and light can also help you tackle jet lag and daytime sleepiness. If you find yourself incredibly tired during the day, because of jet lag or for whatever other reason, a short walk in the sunshine helps remind your body that it’s daytime and keeps you awake. Similarly, when you do go to bed, keeping your room as dark as possible helps cue melatonin to help you drift off.
According to the National Sleep Foundation, two other hormones linked to sleep are:
- Growth hormone (somatotropin). This stimulates growth, cell regeneration, and cell reproduction, and is typically released during NREM sleep. “In children and young adults, the most intense period of growth hormone release is shortly after the onset of deep sleep,” Colorado State University says.
- Cortisol, a hormone that is released in response to stress. Research suggests that sleep deprivation is linked to higher cortisol levels — associating sleep deprivation with “immune compromise, cognitive impairment, and metabolic disruption,” the Dartmouth Undergraduate Journal of Science says.
Have you ever heard that a lack of sleep can cause you to get sick? It turns out that it may not be a myth. “Studies show that people who don’t get quality sleep or enough sleep are more likely to get sick after being exposed to a virus, such as a common cold virus,” the Mayo Clinic says. “Lack of sleep can also affect how fast you recover if you do get sick.”
Harvard Healthy Sleep explains it like this: “Sleep deprivation increases the levels of many inflammatory mediators, and infections, in turn, affect the amount and patterns of sleep. While scientists are just beginning to understand these interactions, early work suggests that sleep deprivation may decrease the ability to resist infection.”
If you are immunocompromised or going through something that affects your immune system, such as chemotherapy, experts say you should try to prioritize sleep. “Getting an adequate amount of sleep is an essential requirement for good health,” the Immune Deficiency Foundation says. “Most scientists recommend a consistent number of hours of sleep per night and consistent bed times and waking times, as well.”
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If you find yourself sick with a cold or stomach bug, sleep is essential to recovery. “Getting plenty of rest is arguably the most important strategy when it comes to bouncing back from an illness,” the American College of Healthcare Sciences says. “Aim to add at least an extra hour to your normal sleep time whenever you feel illness creeping in, either by going to bed earlier or sleeping in later (or ideally both). If possible, sneak in a few small naps during the day to give your body a break and provide it with additional opportunities for rejuvenation.”
Organs & Muscles
Have you ever been drifting into sleep, only to experience a whole-body jolt that shakes you awake? This is called a hypnagogic jerk or a “sleep start,” and it’s a type of muscle spasm. These are totally normal, though experts aren’t totally sure what causes them. “Persons with sleep myoclonus are rarely troubled by, or need treatment for, the condition,” the Baylor College of Medicine website explains. “However, myoclonus may be a symptom in more complex and disturbing sleep disorders, such as restless legs syndrome, and may require treatment by a doctor.”
Sleep is important for muscle growth and muscle repair: You need sleep to recover from strength training and to build more muscle. Sleep is also crucial for athletic performance, reducing injury risks and improving speed, accuracy, and stamina.
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The Cleveland Clinic explains that, during rapid eye movement (REM) sleep, your muscles are completely paralyzed: “The face, fingers, and legs might twitch. Intense dreaming occurs during REM sleep as a result of heightened cerebral activity, but paralysis occurs simultaneously in the major voluntary muscle groups.”
Even though your eyes are closed while you are asleep, your eyes do move around (behind your eyelids) during REM sleep. One theory is that each eye movement is associated with a new “vision” or physical scene in your dream. Plus, some people do actually sleep with their eyes open — the American Academy of Ophthalmology says this can be caused by eyelid problems or issues with the facial nerves: “In most cases, the lids will close most of the way but not completely. Even a small opening in the eyelids can dry out the eyes overnight.” If you wake up with eye dryness, red eyes, blurry vision, or feeling like something is stuck in your eye, you might be sleeping with your eyes partly open.
While sleeping, your major organs continue to function as they normally would:
Your heart rate can drop to quite a low level during deep sleep, the American Heart Association says, but it continues to pump blood around your body while you are resting. However, things can speed up during REM sleep. “Your blood pressure and heart rate can go up and down during this stage,” the UCLA Health website says. “If you have a nightmare that wakes you up, you may find that your heart is racing.” If you wear a smartwatch or heart monitoring device while you sleep, you may notice what heart rate patterns seem normal for you throughout the course of the night.
Your kidneys continue to filter your blood and create urine. “Kidney function is actually regulated by the sleep-wake cycle. It helps coordinate the kidneys’ workload over 24 hours,” Dr. Ciaran McMullan, M.D., told the National Kidney Foundation. “We also know that nocturnal patterns can affect chronic kidney disease and that people who sleep less usually have faster kidney function decline. What we’re doing now is looking at the specific hormones that may be behind these declines.”
Your lungs keep you breathing while you sleep in order to keep blood oxygen levels steady, but some people suffer from breathing conditions during sleep. One example is sleep apnea, explained a little bit more in the next section. Another is chronic obstructive pulmonary disease (COPD). “Sleep problems and sleepiness are common in COPD patients, partly due to symptoms but also because of the medications used to treat COPD,” the National Sleep Foundation says. “In addition, changes in breathing patterns that occur during normal sleep that do not affect healthy people may lead to more severe consequences in people with COPD, which may worsen and complicate COPD since they reduce blood oxygen.”
Other Crazy Things That Happen
A number of unexpected things can happen when you are sleeping, either due to a medical condition or a complete fluke of luck! These include:
The Mayo Clinic explains that snoring happens when “air flows past relaxed tissues in your throat, causing the tissues to vibrate as you breathe.” A few factors play into this, including the anatomy of your mouth and throat, alcohol consumption, your sleep position, and nasal passage issues. If you find that snoring is waking you up at night or disturbing your partner, you can try sleeping on your side or speaking with a doctor to see if the snoring is associated with obstructive sleep apnea.
Getting out of bed at night and moving around while completely asleep doesn’t just happen in cartoons. In fact, research suggests that sleepwalking in adults is less rare than you might think — it affects around 3.6% of Americans, which is more than 8.4 million adults. “It is thought that medication use and certain psychological and psychiatric conditions can trigger sleepwalking, but the exact causes are unknown,” a Harvard Health report on sleepwalking explained. If you are prone to sleepwalking or live with someone who sleepwalks, there are potential safety risks like falling or accidental injuries — speak with a doctor about how to treat sleepwalking or make your home a safer place for sleepwalkers.
While you might feel like sleep is idle time, that’s actually far from the truth. During sleep, your body is working hard to move oxygen through your body, repair cells, grow and repair muscles, filter your blood, and more.
There are plenty of things happening during sleep that we still don’t totally understand, particularly when it comes to brain activity or dreaming. That said, If you are worried about anything that’s happening while you sleep, you can speak with your doctor about what might be happening and how best to address it. For example, if you consistently wake up experiencing physical discomforts like dry eyes or a sore throat, that may be worth chatting to a medical professional about.
If you want to gather more information about your sleep habits, one good first step is to keep a sleep diary. This can be done in a notebook, using an app, or on a spreadsheet — whatever works best for you. For a few weeks, track what time you go to bed and what time you wake up, and how well you slept. Consider tracking other relevant information like what time you ate dinner, the latest point in the day you consumed caffeine, late-night snacks, whether you used electronics before bed, how stressed you felt that day, whether you took a nap at any point and anything else you can think of.
Once you have data from a few days or a few weeks, you can look for any patterns — do you toss and turn after eating a heavy meal, or get up to go to the bathroom multiple times per night after consuming alcohol? Knowing your personal patterns can help you decide what changes to make to improve your sleep quality.
As a first step for improving any sleep issues, most physicians will recommend adopting a rigorous sleep hygiene routine. Sleep hygiene is a set of habits that help you drift off quickly and make it more likely that you will routinely get high-quality sleep. Recommended sleep hygiene steps include:
- Avoiding alcohol, caffeine, and heavy meals or heavy snacking close to bedtime
- Avoiding blue light emitted by electronics like phones, tablets, TVs, or computers before at least one hour before bedtime
- Going to bed at the same time every night
- Waking up at the same time every morning — even on weekends
- Establishing a bedtime routine that helps you relax and wind down, like listening to music, reading, stretching, or sipping a cup of herbal tea
- Using your bed for sleep or sex only
- Keeping your bedroom cool, dark, and as quiet as possible (using blackout shades, an eye mask, earplugs, white noise, or a fan as needed)
- Getting regular exercise, ideally earlier in the day so you feel tired at night
- Managing any sources of constant stress or anxiety, so you don’t lie awake worrying when you are trying to go to sleep
- Getting out of bed to read or listen to music once you have laid in bed awake for 20 minutes, and returning to bed once you feel tired again
You can find more sleep hygiene recommendations online, or by speaking with a physician or a certified sleep coach.
- What Happens in the Brain During Sleep? – Scientific American
- What Happens When You Sleep? – National Sleep Foundation
- The Science of Sleep: Understanding What Happens When You Sleep — Johns Hopkins
- Sleep Deprivation and Deficiency — National Heart, Lung, and Blood Institute
- The Benefits of Slumber | NIH News in Health
- The Characteristics of Sleep | Healthy Sleep
- How Does Sleep Work? | Sleep.org
- Sleep Disorders | Cleveland Clinic
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