What Happens to Your Body During Deep Sleep

Medically Reviewed by Sabrina Felson, MD on May 07, 2022

During deep sleep, you pay less attention to the outside world. But while you may be out like a light, some parts of your body are hard at work. Your breathing and heart rate go down, but your ability to fight germs and to form memories goes up.

Experts are still figuring out exactly what deep sleep is for. But they know everyone needs it. A lack of restorative sleep raises your chances of infections, thinking and memory problems, and other health issues.

You have two kinds of sleep: rapid eye movement (REM) and non-rapid eye movement (NREM). Your deepest sleep happens in stage 3 of NREM. It’s also called N3 or slow-wave sleep (SWS). You’ll get most of it in the first half of the night.

Your body processes change depending on what stage of sleep you’re in.  Here’s what happens during slow-wave sleep:

Temperature. Your brain and body cool down. You make and retain less heat. One theory is that this helps you conserve and restore energy. It’s also a cue from your internal body clock, or circadian rhythm, that it’s time to sleep.

Blood pressure and heart rate. Your parasympathetic nervous system kicks in. That’s sometimes called your “rest and digest” network. Your heart rate slows, and you breathe at a slow and steady pace. Your blood pressure also drops. It’s normal to see a 10%-20% dip.

Bone and muscle. Your pituitary gland sends out human growth hormone. That helps your body repair muscle and other tissue. If you’re a kid, deep sleep will send out growth hormone to help your bones  grow.

Metabolism. You use less energy. Experts think that’s so your body gets a chance to recover from the day. Deep sleep seems to be important for glucose regulation in adults. Studies show a lack of it can lower your insulin sensitivity.  That means you won’t be able to use glucose, or sugar, as well. You might eat more because you’re hungrier.

Energy levels. With good quality sleep, you should feel less tired when you wake up. Your adenosine triphosphate (ATP) levels go up during deep sleep. ATP is a source of energy for your cells. Some experts think this surge is what restores your energy. But we need more research to know for sure.

You usually get tired when you’re sick (and are likelier to get sick if you don't get enough sleep). Experts think that’s because sleep jump-starts your immune system. Research shows you get more deep sleep when you have an infection. If you rest, you may get better faster.

But deep sleep doesn’t just help you when you’re sick. It can:

  • Strengthen your immune system daily
  • Lower chemicals that cause inflammation
  • Help vaccines work in your body

Your brain waves change during this sleep cycle. They get slower, longer, and follow a pattern. These are called delta waves. They’re an important part of how you learn and store memories.

There’s also less blood flow to your brain. That sounds scary, but it isn’t a bad thing. It leaves room for more cerebrospinal fluid (CSF) to flow in and out. That’s the liquid that surrounds your brain and spinal cord. Compared to when you’re awake, CSF helps clear out more waste that can hurt your cells. Deep sleep also gets rid of beta-amyloid, a protein linked to Alzheimer’s disease.

We also need more research to know exactly how CSF and deep sleep work together.

In general, poor quality sleep can take a toll on your mental and physical well-being. It’s linked to health conditions like mood disorders, migraines, heart disease, and obesity.

A loss of deep sleep raises your chances of:

  • Dementia and Alzheimer’s disease
  • High blood pressure
  • Heart attack and stroke  
  • Type 2 diabetes

Here are some healthy habits that may help you get more deep sleep:

  • Give yourself enough time to sleep.
  • Get up at the same time every day.
  • Avoid caffeine.
  • Avoid alcohol before bed.
  • Exercise regularly, but not right before bed.
  • Give meditation a shot.
  • Take a warm bath at least an hour before bed.

Talk to your doctor if you still can’t sleep well. There are treatments that can help.

Show Sources


Roneil Malkani, MD, director, Sleep Medicine Foundation; assistant professor of neurology, Northwestern University.

Physiological Reviews: “The Sleep-Immune Crosstalk in Health and Disease.”

MEDtube Science: “The Neuroprotective Aspects of Sleep.”

Frontiers in Neuroscience: “The Temperature Dependence of Sleep.”

Colten, H., Altevogt, B. Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem, National Academies Press, 2006. 

Harvard Medical School, Healthy Sleep: “The Characteristics of Sleep.”

World Journal of Cardiology: “Night time blood pressure dip.”

Endocrine Development: “Role of Sleep and Sleep Loss in Hormonal Release and Metabolism.”

National Sleep Foundation: “How Sleep Adds Muscle.”

Harvard Health Publishing: “Growth hormone, athletic performance, and aging.”

International Journal of Endocrinology: “Sleep and Metabolism: An overview.”

Sleep: “Does the Brain Gain Back Energy During Sleep? But What Does It Mean?” “Effect of Slow Wave Sleep Disruption on Metabolic Parameters in Adolescents,” “Reduced Slow-Wave Sleep Is Associated with High Cerebrospinal Fluid AB42 Levels in Cognitively Normal Elderly.”

The Journal of Neuroscience: “Sleep and Brain Energy Levels: ATP Changes during Sleep.”

Science: “Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep.”

Journal of Clinical Sleep Medicine: “Slow Wave Sleep: Does it Matter?”

Science Translational Medicine: “Reduced non-rapid eye movement sleep is associated with tau pathology in early Alzheimer’s disease.”

Hypertension: “Decreased Slow Wave Sleep Increases Risk of Developing Hypertension in Elderly Men.”

Proceedings of the National Academy of Sciences of the United States of America: “Slow-wave sleep and the risk of type 2 diabetes in humans.”

National Institute of Neurological Disorders and Stroke: “Brain Basics: Understanding Sleep.”

PLOS One: Short Meditation “Trainings Enhance Non-REM Sleep Low-Frequency Oscillations.”

© 2022 WebMD, LLC. All rights reserved. View privacy policy and trust info