Sleep was long considered just a uniform block of time when you are not awake. Thanks to sleep studies done over the past several decades, it is now known that sleep has distinct stages that cycle throughout the night in predictable patterns. How well rested you are and how well you function depend not just on your total sleep time but on how much of the various stages of sleep you get each night.
Our brain stays active throughout sleep, and each stage of sleep is linked to a distinctive pattern of electrical activity known as brain waves.
Sleep is divided into two basic types: Rapid eye movement (REM)
sleep and Non-REM sleep (with four different stages).
Non-REM Sleep
Stage 1: Light sleep; easily awakened; muscle activity; eye movements slow down.
Stage 2: Eye movements stop; slower brain waves, with occasional bursts of rapid brain waves.
Stage 3: Considered deep sleep; difficult to awaken; brain waves slow down more, but still have occasional rapid waves.
Stage 4: Considered deep sleep; difficult to awaken; extremely slow brain waves.
REM Sleep
Usually first occurs about 90 minutes after you fall asleep; cycles along with the non-REM stages throughout the night. Eyes move rapidly, with eyelids closed. Breathing is more rapid, irregular, and shallow. Heart rate and blood pressure increase. Dreaming occurs. Arm and leg muscles are temporarily paralyzed.
Medical Education for Undergraduate MD Students 