While you sleep, your body recovers from stress, produces hormones, and builds new tissue. At the same time, your metabolism slows down and your heart rate decreases. The brain undergoes quite a lot of changes, too.
There’s a reason why health experts recommend at least eight hours of sleep per night.
The moment you hit the sack, your brain and body work together to balance your hormones, restore your energy levels, and build new neural pathways. These processes have a direct impact on your mood, immune system, and overall health.
Eager to find out more? Here’s what happens in your body and brain when you sleep—and why it matters.
Understanding the Stages of Sleep
First of all, let’s take a quick look at the two stages of sleep: REM (rapid eye movement) and non-REM (non-rapid eye movement) sleep. Your brain and body undergo a number of changes during each of these phases.
- When you close your eyes and fall asleep, your muscles relax and eye movements slow down. At the same time, your brain becomes less responsive to its surroundings.
This stage is called non-REM sleep and accounts for about 80% of your total sleep time, explains the Sleep Disorder Support Foundation (SDSF).
- Next is REM sleep, a state characterized by rapid eye movements, increased heart rate, and higher brain activity. Your muscles become stiffer, blood pressure goes up, and breathing becomes irregular and shallow. This phase makes up around 20% of total sleep time. That’s when most dreams occur.
Non-REM sleep has three distinct phases, namely N1, N2, and N3. During these stages, your body makes the transition from wakefulness to deep sleep.
Stage N1
Stage N1 accounts for approximately 5% of total sleep time, notes the SDSF. That’s about five to 10 minutes, according to the Cleveland Clinic. While in this state, you’ll experience the following:
- Eye movement, heartbeat, brain wave activity, and muscle activity slow down
- Your muscles relax, but they may twitch every now and then
- Breathing rate slightly decreases
Stage N2
Stage N2 makes up about 55% of sleep time. During this phase, your eye movement slows. There’s also a gradual decrease in brain wave activity, heart rate, and body temperature. Your blood pressure may drop as well.
Stage N3
Stage N3, which accounts for 20% of total sleep time, offers the deepest and most restful sleep. This phase is characterized by the following changes:
- A marked decrease in blood pressure
- Slower breathing
- Increased blood flow to the muscles
- Decreased body temperature
- An increase in delta brain waves
- Your body produces hormones, builds bone and muscle tissue, and restores its energy
During the night, your body goes through both non-REM and REM sleep and then starts all over. A full sleep cycle lasts anywhere between 90 and 110 minutes, says the Cleveland Clinic.
As the night progresses, the period of REM sleep increases, while deep (non-REM) sleep becomes shorter.
What Happens When You Sleep?
Now that you understand the changes you’re going through while asleep, you may be wondering why it matters.
For starters, both your brain and body are active during sleep.
The mental and physical processes described above allow you to learn new things, process emotions, and consolidate memories. They also promote cellular regeneration and keep your immune system strong.
Sleep also regulates your mood, appetite, and alertness, points out the SDSF. For example, the pituitary gland produces growth hormone (GH) at night. GH plays a key role in muscle growth and repair, metabolism, and bone development.
So, what happens when you sleep? Let’s find out.
#1. Your Brain Consolidates Memories and Stores Information
Human and animal brains remain active during sleep, forming long-term memories. The information acquired throughout the day is stored in the hippocampus and then transferred to the cortex, according to Science Daily.
Deep sleep, in particular, contributes to learning. However, this doesn’t mean your brain stores every single piece of information.
First, it evaluates information based on future expectations, such as how useful you’ll find it at a later date, and then retains the most relevant memories, explains Science Daily.
#2. Sleep Activates Immunological Memory
Like your brain, the immune system can form memories while you’re asleep. This process is known as immunological memory and has the role to protect you against bacteria, viruses, and other pathogens.
Sleep activates certain immune responses that allow your body to recognize and fight harmful agents, notes a 2013 research paper published in Physiological Reviews.
Simply put, if you are exposed to bacteria or viruses, memory T and B cells will remember that information. As a result, your immune system will respond more rapidly to the threat, which in turn may lower your risk of disease.
A good night’s sleep also has beneficial effects on the formation of adaptive immune responses.
In one study, subjects who slept normally after receiving a vaccine had two times more antigen-specific antibodies four weeks later compared with those who didn’t sleep the first night after getting the vaccine, reports the above review.
#3. Hormones Flood Your Body
The human body produces several key hormones at night, including melatonin, ghrelin, leptin, and growth hormone.
Ghrelin and leptin, for example, regulate appetite. Leptin induces satiety, while ghrelin stimulates appetite and food intake.
Their levels in the bloodstream increase during sleep, according to a 2011 review published in Endocrine Development.
Considering these aspects, it’s not surprising that obesity and sleep deprivation are strongly connected. As the researchers note, children and adults who don’t get enough rest are more likely to gain weight in the long run. The risk of obesity is highest among those who sleep less than six hours per night.
Your body also releases growth hormone (GH) during the night. GH reaches peak levels at the onset of deep sleep.
This hormone regulates protein, fat, and glucose metabolism, as well as insulin secretion and bone growth. Due to its role in energy metabolism, it has a direct impact on body composition, or fat-to-muscle ratio.
Melatonin, another hormone secreted during sleep, regulates your body’s internal clock. When produced in adequate amounts, it makes it easier to fall asleep and promotes quality rest.
#4. Your Stress Levels Decrease
Cortisol, the so-called stress hormone, affects your mood, alertness, and mental focus. It also plays a role in metabolism, digestion, and immune function.
Daily stress, traumatic events, and other factors can skyrocket cortisol levels, leading to depression, anxiety, and poor health. One way to bring them down is to get more rest.
As Healthline notes, cortisol levels drop around midnight and increase around 9 A.M.
Sleep deprivation may cause your body to produce more cortisol during the day. This may increase your risk of weight gain, memory problems, heart disease, and mental disorders.
High cortisol levels also trigger inflammation, which is a contributing factor to chronic pain, cardiovascular problems, Alzheimer’s disease, and other illnesses.
#5. Your Immune System Releases Anti-Inflammatory Cytokines
Adequate sleep not only boosts immunological memory, but may also reduce the inflammatory response.
When you’re asleep, your immune system releases cytokines that fight inflammation, pathogens, and infectious diseases, explains the Mayo Clinic.
Clinical research shows that sleep deprivation may increase the risk of infectious or inflammatory diseases. When you don’t get enough sleep, your body becomes less efficient at fighting bacteria and other disease-causing microorganisms.
Improper rest also triggers low-grade inflammation. Over time, this condition can become chronic and take a toll on your health.
#6. Your Body Repairs Damaged Tissues
Sleep is primarily a restorative process, points out the College of William & Mary. During the night, your body repairs cells, tissues, and organs while producing testosterone and other hormones that aid in recovery.
Elite athletes hit the sack early to make sure they get enough sleep—and for good reason. A good night’s sleep allows them to recover faster from training and perform at their peak.
LeBron James, for example, sleeps about 12 hours per night. Other top athletes, such as Usain Bolt and Steve Nash, get at least 10 hours of rest per day. Maria Sharapova sleeps eight to 10 hours each night, while Lindsey Vonn gets at least nine hours of shuteye.
Adequate sleep allows you to recover from mental and physical stress. Your body uses this time interval to build and repair muscle, synthesize protein, and restore its energy. If you’re ill or injured, a good night’s sleep can help you get back on your feet faster.
#7. Your Heart Slows Down
Your heartbeat, breathing, digestion, and other bodily functions all slow down at night. Heart rate, for example, decreases by about 20% to 30% during deep sleep, reports Harvard Medical School.
However, note that large body movements and certain changes in brain wave activity may cause sharp increases in heart rate. These events may explain why the risk of myocardial infarction is higher in the morning.
Your resting heart rate should stay between 60 and 100 beats per minute, regardless of whether you’re awake or sleeping. Consider using a sleep tracker to monitor your heart rate at night. Reach out to your doctor if you notice anything unusual.
#8. Your Muscles Paralyze
The most vivid dreams occur when you’re in REM sleep. This is also when the muscles that are under conscious control lose their ability to move, explains Live Science. Basically, they’re temporarily paralyzed.
This process has the role of keeping your body still during deep sleep. Otherwise, you would end up acting out your dream with involuntary movements like punching, laughing, crying, or talking aloud.
#9. Your Metabolic Rate Decreases
As discussed earlier, your metabolism slows down when you’re asleep. This allows your body to conserve energy. Think of it as a survival mechanism.
A 2011 study published in the Journal of Physiology found that total daily energy expenditure (EE) increases during periods of sleep deprivation and goes down during rest.
To save energy, your body lowers its core temperature, heart rate, blood pressure, and other functions.
#10. Saliva Flow Slows Down
Ever wonder why you have a dry mouth at night? One reason is that saliva flow decreases during sleep, as there is nothing to digest.
Your body still produces small amounts of saliva during the night to protect the oro-esophageal structures and other tissues. However, saliva flow is significantly lower at night than during the waking hours of the day.
If your mouth is constantly dry during sleep, take a few sips of water before bedtime or throughout the night (in case you wake up). Consider using a humidifier to prevent excessive dry air in the bedroom.
#11. Your Brain Generates Dreams
Until recently, scientists believed that dreaming only occurs during REM sleep. According to the latest evidence, this process also takes place during non-REM sleep. Even today, there’s a lot of debate regarding the purpose of dreams.
Some researchers say that dreaming is a coping mechanism. For example, they may help you confront emotional traumas and face your fears, explains Healthline.
Others believe that our brain generates dreams as part of the fight-or-flight response. Simply put, they may help us prepare emotionally for a potential threat.
Dreams may also reflect our creative tendencies, emotions, and day-to-day experiences. Freud believed that they’re a key to access in our inner world.
Other theories suggest that dreaming helps consolidate memories, filters out information, or enables the brain to replay and analyze recent events.
Your Body Does Incredible Things When You’re Asleep
Now that you know what happens when you sleep, it’s easy to understand why you need to prioritize rest.
When you’re asleep, your body builds muscle, produces hormones, fights inflammation, and repairs damaged cells. Your brain stores and processes information, consolidates memories, and regenerates itself.
A good night’s sleep can boost your memory, learning abilities, and creative skills. It also supports immune function and keeps your stress levels in check. Plus, it energizes your mind and body, preparing you for the day ahead.
Remember that every hour of sleep counts. Even if you’re a night owl, it’s never too late to fix your sleep schedule and reap the benefits of proper rest.
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