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.

Deep Sleep and Your Body

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.

Deep Sleep and Your Immune System

You usually get tired when you’re sick. 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

Deep Sleep and Your Brain

Your brain waves change during this sleep cycle. They get slower, bigger, 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. CSF seems to hitch a ride on your delta waves. Compared to when you’re awake, that 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.

Risks of Too Little Deep Sleep

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

How to Get More Deep Sleep

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.
  • Don’t drink caffeine late at night.
  • 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.

WebMD Medical Reference

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