If you are getting the recommended amount of slumber — seven to nine hours a night — you’re spending about one-third of your life asleep.

Although that may seem like a lot of time, your mind and body are very busy during that time, so that you can be productive, energetic, and healthy when you’re awake.

There are five stages of sleep that rotate between non-rapid eye movement (NREM) and rapid eye movement (REM) and include drowsiness, light sleep, moderate to deep sleep, deepest sleep, and dreaming.

Sleep stages 1, 2 and REM consist of light sleep, while 3 and 4 comprise deep sleep.

Stage 1

During stage 1, you drift from being awake to being asleep. This is a light, NREM sleep that doesn’t last very long. You may start to relax and dream, but may also twitch as you transition into stage 2.

Stage 2

Stage 2 of the sleep cycle is still a light sleep, but you are drifting into a steadier sleep. Your breathing and heartbeat slow down, and your muscles relax. Your body temperature decreases, and your brain waves are less active.

Stages 3 and 4

In stage 3, you enter deep sleep, and stage 4 is the deepest sleep stage. During deep sleep, your breathing, heartbeat, body temperature, and brain waves reach their lowest levels. Your muscles are extremely relaxed, and you are most difficult to rouse.

Stage 4 is known as the healing stage, when tissue growth and repair take place, important hormones are released to do their jobs, and cellular energy is restored.

REM sleep

Your first REM cycle of the night begins about 90 minutes after you fall asleep and recurs every 90 minutes. Your eyes move around quickly behind your eyelids and your brainwaves look similar to those of someone who is awake. Your breathing, heart rate, and blood pressure rise to near-waking levels.

REM sleep, often referred to as stage 5, is when you are most likely to dream.

Your arms and legs become temporarily paralyzed during this stage to prevent you from physically acting out your dreams.

In healthy adults, about 13 to 23 percent of your sleep is deep sleep. So if you sleep for 8 hours a night, that’s roughly 62 to 110 minutes.

However, as you get older you require less deep sleep.

During deep sleep, memories are consolidated, learning and emotions process, physical recovery takes place, blood sugar levels and metabolism balance out, your immune system is energized, and your brain detoxifies.

Without deep sleep, these functions cannot take place and the symptoms of sleep deprivation kick in.

On the other hand, there doesn’t seem to be any such thing as too much deep sleep.

How much REM sleep should you get

Although there is no official consensus on how much REM sleep you should get, dreaming is most common during this stage and experts believe that dreaming helps you process emotions and solidify certain memories.

For most adults, REM takes up about 20 to 25 percent of sleep, and this seems to be healthy during average sleep cycles. If people get excessive amounts of REM, however, they may be more likely to suffer from depression.

How much light sleep do you need?

Although sleep scientists believe that light sleep is good for you, there is no minimum to strive for. Light sleep is usually the default stage, one that is nearly impossible to avoid if you are asleep at all.

Too much overall sleep on a regular basis, however, is linked to obesity, depression, pain, heart disease, and even increased risk of death.

How much deep and light sleep do children need?

Babies and children need more sleep than adults. Babies need the most, spending about 16 of every 24 hours asleep. Approximately 50 percent of their slumber is spent in the REM stage, while the other 50 percent is divided between stages 1 through 4 and NREM sleep that cycles between light and deep.

Toddlers need 11 to 14 hours of sleep; preschoolers, 10 to 13; school-aged children should get 9 to 12; and teens require 8 to 10 hours of sleep.

With enough sleep that appears to be restful, it is likely that the light, deep, and REM ratio is exactly where it should be in young people.

If they are having trouble with falling asleep, staying asleep, or sleeping well, or if they are sleeping way too much for their age, children may be irritable, could have learning and memory problems, or they may be more susceptible to illness.

If you sleep eight hours but toss and turn all night, you may not be getting enough deep sleep.

It’s impossible to force your brain to go into deep sleep, but there are a number of strategies that have shown some promise in terms of increasing your percentage of deep sleep. These include:

Although the science is still new, a number of sleep trackers are available that may help you track your sleep patterns and see how much light, REM, and deep sleep you are getting.

According to the American Sleep Apnea Association, you should feel fresh and alert when you wake up, but many people don’t.

If you are sleeping for seven to nine hours each night, but only 10 percent of that is deep sleep, you are not getting the 90 minutes you need and might still be tired each day. A sleep study may help you figure out what is going on.

There are a number of possible causes that you might want to discuss with a doctor, including:

Scientists say that quality sleep is as important to health as food and water are. It helps you to survive and thrive. Some of the side effects of sleep deprivation include:

  • memory troubles
  • mood changes
  • weakened immunity
  • trouble concentrating
  • poor response time and increased risk of accidents
  • high blood pressure
  • weight gain
  • risk for diabetes
  • low sex drive
  • risk of heart disease
  • poor balance
  • early aging

Scientists agree that sleep is essential to health, and while stages 1 to 4 and REM sleep are all important, deep sleep is the most essential of all for feeling rested and staying healthy.

The average, healthy adult gets roughly one to two hours of deep sleep per eight hours of nightly sleep. There are various ways to gauge whether you are, from personal trackers to a sleep study.

If you are waking up tired on a regular basis, it’s a good idea to talk to a doctor.