What Happens to Your Body When You Sleep?

Medically Reviewed by Nayana Ambardekar, MD on March 13, 2021
3 min read

You may be resting when you sleep, but there’s a lot going on behind the scenes. What’s happening varies depending on which part of sleep you’re in -- rapid eye movement (REM sleep) or non-REM sleep -- and even which stage of non-REM sleep you’re in.

It tends to go up and down a little during the day, and the same is true at night, although while you’re sleeping it can be 1 to 2 degrees lower than in the daytime. Body temperature starts to fall as bedtime approaches, paving the way for a good night’s sleep. Your body also tends to lose heat, which helps you fall and stay asleep. That’s one of the reasons experts say you shouldn’t exercise close to bedtime: Exercise heats you up. We sleep better when we’re cooler. Your temperature starts to rise toward morning, preparing your body for wakefulness.

During the day, your breathing changes a lot. It all depends on what you’re doing and feeling. During non-REM sleep (about 80% of an adult’s sleeping time), you breathe slowly and regularly. But during REM sleep, your breathing rate goes up again. That’s the time we typically dream. Breathing also becomes more shallow and less regular during this sleep phase. Some of it may be due to throat muscles relaxing. It may also be due to less movement of the rib cage during REM sleep. Whenever you’re sleeping, your oxygen levels are lower and your carbon dioxide levels are higher because your level of breathing goes slightly down.

Most people don’t cough much while they’re asleep, especially not during REM sleep. Sleep shuts down your cough reflex. If you do cough while asleep, chances are you’re not getting good rest. It may also be a sign of a sleep disorder. A chronic cough may be a symptom of obstructive sleep apnea. That’s when your throat muscles relax and block your airway for brief periods of time. If you think you have this, see your doctor. They can suggest treatments to help.

Just like breathing, your heart rate and blood pressure are different during sleep. And they change depending on what phase of sleep you’re in. Heart rate and blood pressure go down and are steadier during non-REM sleep. During REM sleep, they rise and are more varied, similar to daytime patterns. Changes in blood flow during this sleep period can also cause sexual responses (erections in men and an engorged clitoris in women). As daybreak approaches, both heart rate and blood pressure inch back up. Your chance of having a heart attack is higher at this time.

It’s basically naptime for the nerve cells in your brain as you dip into non-REM sleep. They do send out a few messages, but nothing much. But like so many other bodily functions, brain activity goes up during REM sleep, sometimes even more than during the day. Blood flow to the brain and the metabolism in your brain also go up during REM sleep.

During sleep, the brain limits physical movement. It keeps you from acting out on your dreams. Flailing your arms and legs around while you’re sleeping could be dangerous.

Your brain also uses your sleep cycles to consolidate memories. So staying up all night to cram for a test might be counterproductive.

Your body is busy repairing cells and finishing digestion.

During a good night’s rest, you may not get up to go to the bathroom. That’s because your kidneys make less pee while you sleep.

Growth hormone production surges. Your body makes more thyroid hormones.Levels of cortisol, sometimes called the “stress hormone,” go down when you first fall asleep, then go up again right before you wake up. Levels of melatonin, one of the main chemicals involved in the sleep-wake cycle, do just the opposite: they rise to make you sleepy when the sun sets and ebb at daylight.