Stage 2: Light Sleep
During this stage, your heart rate slows and your body temperature drops. Lasting about 20 minutes, your body is getting ready for deep sleep.
Stage 3: Deep Sleep
During the deep stage of Non-REM sleep, the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system. During this stage, it is harder to rouse someone and, if woken up, you may feel groggy and disoriented for several minutes. Deep sleep is important for feeling refreshed in the morning and if this stage is too short, your sleep will not feel satisfying.
Stage 4: REM Sleep
REM sleep, which usually is entered approximately 90 minutes after falling asleep, is characterized by eye movement, quickened breathing and increased brain activity. REM sleep is also referred to as paradoxical sleep because while the brain and other body systems become more active, muscles become more relaxed. Dreaming occurs because of increased brain activity, but voluntary muscles become paralyzed.