Rapid Eye Movement, REM, sleep is the fourth and final stage of each sleep cycle. This stage is the closest to being awake that you can be, partly due to the amount of eye movement that happens. Your heart beats faster and you breathe faster because this is the stage where dreams happen.
It's important to understand sleep cycles. A good night of sleep has anywhere from four to six sleep cycles that last 90-110 minutes. There are four stages of sleep, the first three known as non-REM; NREM1, NREM2, NREM3. Then REM is the final part before we begin the cycle all over again.