You may have heard that adults need between 7 and 9 hours of sleep each night. But, the quality of sleep you get also matters.

While you rest, your body goes through different stages of the sleep cycle. Deep sleep, for example, is the stage of sleep you need to feel refreshed when you wake up in the morning. Unlike rapid eye movement (REM) sleep, deep sleep is when your body and brain waves slow down.

It’s hard to wake from deep sleep, and if you do, you may feel particularly groggy.

Read on to learn more about this part of your sleep cycle.

Sleep is divided into two categories: REM and non-REM sleep. You begin the night in non-REM sleep followed by a brief period of REM sleep. The cycle continues throughout the night about every 90 minutes.

Deep sleep occurs in the final stage of non-REM sleep.

Non-REM sleep

Stage 1 of non-REM sleep lasts several minutes as you move from being awake to being asleep.

During stage 1:

  • your body functions — like heartbeat, respiration, and eye movements — begin to slow
  • your muscles relax with only occasional twitches
  • your brain waves start to slow down from their wakeful state

Stage 2 accounts for about 50 percent of the total sleep cycle. This is the stage of sleep you may fall into more than any other throughout the night.

During stage 2:

  • your body’s systems continue to slow and relax
  • your core temperature drops
  • your eye movements stop
  • your brain waves are slow, but you have some short bursts of activity

Stages 3 and 4 are when you experience deep sleep.

During these stages:

  • your heartbeat and breathing become their slowest as your muscles relax
  • your brain waves become the slowest they’ll be while you’re asleep
  • it’s difficult to awaken even with loud noises

Deep sleep is also referred to as “slow wave sleep” (SWS) or delta sleep.

The first stage of deep sleep lasts anywhere from 45 to 90 minutes. It lasts for longer periods in the first half of the night and becomes shorter with each sleep cycle.

REM sleep

Stage 5, or your first stage of REM sleep, occurs about 90 minutes after moving through non-REM stages.

During this stage:

  • your eyes move rapidly from side to side
  • you experience dreaming as your brain activity increases to a more wakeful state
  • your heart rate increases to near its wakeful state
  • your breathing becomes faster and even irregular at times
  • your limbs may even become paralyzed

Glucose metabolism in the brain increases during deep sleep, supporting short-term and long-term memory and overall learning.

Deep sleep is also when the pituitary gland secretes important hormones, like human growth hormone, leading to growth and development of the body.

Other benefits of deep sleep include:

  • energy restoration
  • cell regeneration
  • increasing blood supply to muscles
  • promoting growth and repair of tissues and bones
  • strengthening the immune system

Deep sleep is responsible for helping process the information you encounter each day. Without enough, the brain can’t convert this information to your memory.

Not getting quality sleep is also linked to conditions, like:

The deep sleep stage itself is associated with certain disorders, like:

You spend roughly 75 percent of your night in non-REM sleep and the other 25 percent in REM sleep. Of this, around 13 to 23 percent of your total sleep is deep sleep.

That said, deep sleep decreases with age. If you’re under age 30, you may get two hours of deep sleep each night. If you’re over age 65, on the other hand, you may only get a half hour of deep sleep each night, or none at all.

There’s no specific requirement for deep sleep, but younger people may need more because it promotes growth and development. Older people still need deep sleep, but not getting as much doesn’t necessarily indicate a sleep disorder.

If you wake up feeling exhausted, it may be a sign that you’re not getting enough deep sleep.

At home, wearable devices measure sleep by tracking your body’s movements during the night. This technology is still relatively new. While it may help identify sleep patterns, it may not be a reliable indicator of how much deep sleep you’re getting.

Your doctor may recommend a sleep study called a polysomnography (PSG). During this test, you’ll sleep at a lab while hooked up to monitors that measure:

  • breathing rate
  • oxygen levels
  • body movements
  • heart rate
  • brain waves

Your doctor can use this information to see if you’re reaching deep sleep and other stages throughout the night.

Heat may promote more slow wave sleep. For example, taking a hot bath or spending time in a sauna before bed may help improve your sleep quality.

Eating a low-carbohydrate diet or taking certain antidepressants may also promote deep sleep, though more research is needed in this area.

Getting enough sleep in general may also increase your deep sleep.

Here’s some tips:

  • Put yourself on a bedtime schedule where you go to sleep and wake up at the same time each day.
  • Get plenty of exercise. About 20 to 30 minutes each day is a good start, just avoid working out in the hours before bedtime.
  • Stick to water and other decaffeinated drinks before bed. Caffeine, alcohol, and nicotine may make it harder to get a good night’s rest.
  • Create a bedtime routine to unwind from the day, like reading a book or taking a bath.
  • Banish bright lights and loud noises from your bedroom. Too much TV or computer time may make it hard to relax.
  • Don’t lay in bed tossing and turning. Consider getting up and doing a light activity, like reading, until you’re tired again.
  • Consider replacing your pillows if you’ve had them for over a year and have trouble getting comfortable.

If the above tips don’t help, make an appointment to see your doctor.