The main difference between NREM and REM sleep is the neural and physical activity of the body. The names non-rapid and rapid eye movement indicate the behavior during each stage. During the NREM phases, the brain experiences activity known as sleep spindles. The third NREM phase is also when we experience deep sleep. The REM phase is when we tend to dream and when most of the body is paralyzed. A REM calculator can be used to determine what is the best time to wake up without feeling disoriented.