SLEEP STAGES & SLEEP CYCLE

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 WHAT ARE THE STAGES OF SLEEP?
  When you sleep, your brain goes through natural cycles of activity. There are four total stages of sleep, divided into two phases:
  • Non-REM sleep happens first and includes three stages. The last two stage of non-REM sleep is when you sleep deeply. It’s hard to wake up from this stage of sleep.
  • REM sleep happens about an hour to an hour and a half after falling asleep. REM sleep is when you tend to have vivid dreams.
  As you sleep, your body cycles through non-REM and REM sleep. You usually start the sleep cycle with stage 1 of non-REM sleep. You pass through the other stages of non-REM sleep, followed by a short period of REM sleep. Then the cycle begins again at stage 1. A full sleep cycle takes about 90 to 110 minutes. Your first REM period is short. As the night goes on, you’ll have longer REM sleep and less deep sleep.

What Is the Sleep Cycle?

  Sleep is not uniform. Instead, over the course of the night, your total sleep is made up of several rounds of the sleep cycle, which is composed of four individual stages. In a typical night, a person goes through four to six sleep cycles1. Not all sleep cycles are the same length, but on average they last about 90 minutes each.

Are All Sleep Cycles the Same?

  It is normal for sleep cycles to change as you progress through your nightly sleep. The first sleep cycle is often the shortest, ranging from 70-100 minutes, while later cycles tend to fall between 90 and 120 minutes. In addition, the composition of each cycle — how much time is spent in each sleep stage — changes as the night goes along.

  Sleep cycles can vary from person to person and from night to night based on a wide range of factors such as age, recent sleep patterns, and alcohol consumption.

What is non-REM sleep?

Three stages make up non-REM sleep.

Stage 1: This stage of light sleeping lasts for five to 10 minutes.
· Everything starts to slow down, including your eye movement and muscle activity.
· Your eyes stay closed. If you get woken from stage 1 sleep, you may feel as if you haven’t slept at all. You may remember pieces of images.
· Sometimes, you may feel like you’re starting to fall and then experience a sudden muscle contraction. Healthcare providers call this motion hypnic myoclonic or hypnic jerk. Hypnic jerks are common and not anything to be concerned about as this occurrence is unlikely to cause any complications or side effects.
Stage 2: This period of light sleep features periods of muscle tone (muscles partially contracting) mixed with periods of muscle relaxation.
· Your eye movement stops, heart rate slows and body temperature decreases.
· Brain waves become slower. Occasionally, you’ll have a burst of rapid waves called sleep spindles.
· Your body prepares to enter deep sleep.
Stages 3: This stage is deep sleep. During this stage, your brain produces delta waves, very slow brain waves.
·  It’s hard for someone to wake you up during this stage.
· You have no eye movement or muscle activity.  
·  If you’re woken up, you may feel groggy and disoriented for a few minutes.
What happens during non-REM sleep?
During non-REM stages, your body:
·  Builds bone and muscle.
·  Repairs and regenerates tissues.
·  Strengthens the immune system.
As you age, you get less non-REM sleep. Older adults get less deep sleep than younger people.
What is REM sleep?
   When you enter REM sleep, brain activity increases again, meaning sleep is not as deep. The activity levels are like when you’re awake. That’s why REM sleep is the stage where you’ll have intense dreams. At the same time, major muscles that you normally control (such as arms and legs) can’t move. In effect, they become temporarily paralyzed.
   Usually, REM sleep arrives about an hour and a half after you go to sleep. The first REM period lasts about 10 minutes. Each REM stage that follows gets longer and longer. The amount of REM sleep you experience changes as you age. The percentage of REM sleep:
· Is highest during infancy and early childhood.
· Declines during adolescence and young adulthood.
· Declines even more as you get older.
What else happens to the body in REM sleep?
  Besides increased brain activity and muscle relaxation, your body goes through a series of changes during REM sleep. These changes include:
· Faster breathing.
· Increased heart rate and blood pressure.
· Penile erections.
· Rapid eye movement.






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