What Happens When You Sleep? (sleep stages)

While you’re asleep each night, your body typically goes through several sleep cycles that last on average 90 minutes. In each cycle you alternate between two types of sleep:
Light Sleep and Deep Sleep—With less brain activity than REM sleep, non-rapid eye movement (NREM) includes the stages of light sleep and deep sleep. Periods of deep sleep are typically longer early in the night.
Rapid Eye Movement (REM) Sleep—The stage associated with vivid dreams, REM sleep periods are typically longer as the night goes on.

WHAT ARE THE STAGES OF SLEEP?
There are five stages of sleep. Stages 1-4 are non-REM sleep, followed by REM sleep.

STAGE 1
INTRODUCTION TO SLEEP: Your eyes fight to stay open. Your muscles begin to relax. The physical world starts disappearing around you.
This first stage of sleep is called the “introduction into sleep”. It is frequently observed by watching someone’s head nod when they are listening to a dull lecture. This stage is marked by a slowing down of brain activity and a beginning of muscle relaxation. You can be easily awoken from this stage, which is why you find yourself jumping awake for seemingly no reason.
Fun Fact: That jump you experience that makes you feel suddenly wide awake is called a myclonic jerk.

STAGE 2
BEGINNING OF SLEEP: Light, dreamless sleep. A relaxation takes over the body to prepare for the dreams that are coming.
This stage of sleep is the official “beginning of sleep”. There is a further slowing down of brain and muscle activity. This stage is best described in the following manner: You have been asleep in bed for 15-30 minutes and the phone rings. While you may regret being bothered in your sleep, you are easily able to answer the phone and, unless you tell the person who called you that you were asleep, they would never know. You become quickly alert and are able to easily engage in conversation.
Fun Fact: In whales and dolphins only one brain hemisphere falls asleep at a time so that they can still surface from the water to breathe. Some people will have enough relaxation of muscles in the throat area in this sleep stage that the upper airway may start collapsing and snoring begins.

STAGE 3&4
SLOW WAVE SLEEP: Deep sleep. Building up physical and mental energy. This is where the body gets rest.
These stages of sleep are collectively called “slow wave sleep”. In slow wave sleep, brain and muscle activity decrease significantly. In our example, you are now asleep in bed for 45 minutes and the phone rings. You fumble for the phone and finally answer it. Anyone calling you would immediately know that they have awakened you. You are groggy and find it difficult to engage in conversation.
Fun Fact: If it takes you less than 5 minutes to fall asleep, then you could sleep deprived. It should take closer to 10 minutes. There is so much relaxation of the muscles in the throat that people who are susceptible to sleep-related breathing disorders can experience difficult breathing.

REM
RAPID EYE MOVEMENT: Rapid eye movement. Dreaming. Improves brain function and creates long term memories.
REM, or rapid eye movement sleep, is the time of sleep when dreaming occurs. During REM sleep the brain is very active, and yet, with the exception of the heart and lungs, the muscles of the body are paralyzed. This muscle paralysis has significant implications for sleep apnea or sleep suffocation sufferers. Since they experience complete muscle relaxation or paralysis in REM sleep, their airways can easily completely collapse. Most importantly, these people have far more difficulty resuming normal breathing after the airway has collapsed.
Fun Fact: A study shows that people who get 6-7 hours of sleep have a longer life expectancy than those who get 8 hours. REM sleep grows longer after about six hours of sleep. This is why you commonly wake up in the middle of a dream.