While asleep, the body moves through a cycle of five different stages: 1, 2, 3, 4 and REM (rapid eye movement) sleep. The sleep cycle lasts an average of 90 minutes and throughout the night, the body goes through this cycle four-six times. Each stage of sleep performs a different restorative function, which is why without a full night of sleep, your body and mind lack the essentials to help you get through the day. Learn about each stage below:

Stage 1: The Lightest Stage
During this phase, you drift in and out of consciousness. Your muscles begin to relax and your brain activity slows down. You still remain somewhat alert and can be easily woken.

Stage 2: Beginning of Sleep
The second stage is also rather light. During this stage, your brain and muscle activity slows down further and your brain produces waves that are called sleep spindles. Your body temperature starts to drop and your heart rate slows down as preparation for deep sleep.

Stages 3 & 4: Slow Wave Sleep
These two stages are deeper stages of sleep. These stages are often the hardest to wake up from and are known as “slow wave sleep” because the brain waves slow to delta waves (extremely slow brain waves). This type of sleep is comprised of the deepest stage of NREM. These phases are extremely rejuvenating to the body.

Stages 5: (REM) Rapid Eye Movement Sleep
Dreaming occurs during this phase. This phase is unlike the previous four phases because the brain is very active while skeletal muscles remain atonic. Your mind energizes itself while your body is immobile. During this phase, your eyes remain closed but move rapidly from side to side. This stage revitalizes the brain to support daytime function.

Article Sources:
https://sleep.org/articles/what-happens-during-sleep/
https://www.sleepassociation.org/about-sleep/stages-of-sleep/
https://www.tuck.com/stages/
http://www.centerforsoundsleep.com/sleep-disorders/stages-of-sleep/