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What Happens in Your Body and Brain While You Sleep

by Sarah DiGiulio /
Image: Empty bedJohn Brecher /

You might think of sleep as the negative time in your day when nothing on your to-do list gets done. Your brain and several other systems in your body see it quite differently.

“Your brain is actually very active during sleep doing important things — it’s not just resting,” says Carl W. Bazil, MD, PhD, the Caitlin Tynan Doyle Profesor of Neurology at Columbia University Medical Center. “And if you don’t get sleep you don’t function on a number of levels the way you should.”

(Everything from learning to your mood to your risk of getting sick and becoming obese can get thrown off kilter.)

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Physiologically sleep is defined as a state our bodies enter into during which brain wave activity changes and our nervous system is less reactive to external stimuli (i.e. we temporarily leave consciousness). But our sleep is not constant throughout the night. We actually cycle through four distinct sleep phases multiple times (five if you count “awake” as one stage), Bazil, who is also Director of the Division of Epilespy and Sleep at Columbia University College of Physicians and Surgeons, tells NBC News BETTER.

There are two stages of light sleep. The lightest is the stage of sleep you’re likely in if you nod off during a lecture when consciousness is decreased, but the brain is still processing some information around you (sometimes hearing your name or another stimulus will jolt you awake). Intermediate light sleep is slightly deeper, which is harder to awaken from, Bazil explains.

Your brain is actually very active during sleep doing important things — it’s not just resting.

Deep slow-wave sleep is the next stage of sleep. This is the deepest, most restful, and most restorative stage of sleep, when it’s hardest to awaken. If you do get woken up during this stage of sleep you’re likely to feel groggy. And finally, there’s REM sleep (short for “rapid eye movement sleep”), which is when we dream. Our bodies tend to spend more time in restful slow wave sleep earlier in the night when our bodies and minds are most tired. Later in the night we tend to spend more time in REM sleep.

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There are important electrical and chemical processes that happen in the brain and throughout the body during all the stages of sleep. Here’s how they affect our health:

Sleep is prime time for learning and memory

One of the most active parts of the body during sleep is the brain, Bazil says. There are pronounced changes in the electrical activity of the brain during sleep, which the evidence suggests is a result of the brain’s trillions of nerve cells literally rewiring themselves. This rewiring, which happens during deep, slow-wave sleep, is how we process and are thus able to retain new information we may have learned throughout the day, Bazil explains. “Your brain is making a map of the information,” he says — “making new connections and breaking other ones.”

That means skipping sleep to cram for an exam or important presentation isn’t doing you any favors, Bazil says. The evidence suggests that if you spend all night trying to learn something new and miss a few hours of sleep to do so, your brain’s not going to retain that information the same way it would have if you’d gotten a full night of sleep, he says. “Your brain really needs to process that information, which you really only do when you’re asleep.”

Sleep also helps keep our attention and focus sharp, Bazil adds. We all (likely) know the “fuzzy” feeling that results after a night of too little sleep, especially if you’re trying to pay attention to a lecture on a complicated topic or focus on a complex task. But it’s also important to note that chronic sleep debt accumulates and research shows the attention and focus deficits caused by sleep loss actually accumulate over time, Bazil explains.

If you spend all night trying to learn something new and miss a few hours of sleep to do so, your brain’s not going to retain that information the same way it would have if you’d gotten a full night of sleep.

One study followed a group of individuals who got six hours of sleep for two weeks. Their attention got progressively worse over that time period and by the end their attention was nearly equivalent to individuals who had been awake for two nights of getting no sleep.

“It’s important for people to know that you can get by with a poor night of sleep,” Bazil says. “But most people need around eight hours of sleep [a night] and if you’re chronically not getting that sleep you need, your performance is going to deteriorate.”

Poor sleep makes you moody

Think cranky toddler in need of a nap. We all know that sleep (and lack of it) affects mood and irritability. But brain-imaging studies have shown that a good night’s sleep helps our brain regulate mood and cope with whatever the next day brings. Conversely, insufficient sleep boosts a part of the brain that’s known to be affected by depression, anxiety and other psychiatric disorders.

“Without sleep, the brain had reverted back to more primitive patterns of activity — in that it was unable to put emotional experiences into context and produced controlled, appropriate responses,” the study’s senior author Matthew Walker, Director of University of California Berkeley’s Sleep and Neuroimaging Laboratory, said in a statement in 2007 (when that research was first published).

Chronic insomnia has also been linked to increased risk of developing a mood disorder, including anxiety or depression. Another study found that after a week of getting just four-and-a-half hours of sleep per night, individuals reported worse moods (in terms of feeling stressed, angry, sad or mentally exhausted).

Not getting sleep can literally make you sick

Outside of the brain, there’s a lot changing throughout the rest of the body during sleep, too. Our heart rate and body temperatures drop, our breathing rate slightly decreases and becomes very regular (at least during most stages of sleep), and kidney function slows down (which is why you typically don’t feel the urge to pee as frequently during sleep as when you’re awake).

And at the same time, other systems in the body ramp way up during sleep. There’s an increase in the release of growth hormones during sleep (this is when kids get taller, our skin cells regenerate, and our hair gets longer), as well as the hormones that regulate appetite. Sleep is also when our muscles repair damage (and regular wear and tear) from throughout the day.

Sleep also plays an integral role in regulating the body’s immune system, which is responsible for fighting off all sorts of problems from the common cold to more serious chronic problems like cancer. (Research suggests that the body produces fewer infection-fighting antibodies when sleep deprived.) Studies have shown that individuals are more likely to catch a cold virus when you’re sleep deprived and that vaccines can be less effective after a poor night of sleep.

And thanks to all these important roles that sleep plays in the body, chronically getting poor sleep can have some pretty serious consequences. Cutting sleep short by even just two to three hours a night over time has been linked to an increased risk of obesity, diabetes, cardiovascular disease, hypertension and premature death.

“There is definitive evidence that food choices are metabolically less favorable at night,” Kristin Eckel-Mahan, PhD, Assistant Professor at the Center for Metabolic and Degenerative Diseases at The University of Texas Health Science Center of Houston, tells NBC News BETTER. “And there is also evidence that the same number of calories eaten at the wrong time can induce increases in body weight, particularly in fat mass.”

Part of this has to do with the fact that insulin sensitivity fluctuates during the day — meaning our bodies actually metabolize food differently at different times of the day, she says. And though there’s a lot more research that’s needed to fully understand the connection between sleep and metabolism, it’s clear that they’re connected, she says — and likely has a lot to do with why people who report getting worse sleep are more likely to be overweight.

The bottom line, says Bazil: sleep is not a waste of time and you can’t get away without it.

When it comes to staying healthy, people pay a lot of attention to nutrition and physical activity, Bazil says — which are both very important. “But I would put sleep on that same level.”

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