Sleep is a special activity of the brain, which is controlled by a sophisticated and precise mechanism. It is a physiological process we cannot live without. Our sleep is divided into four different sleep phases non-REM (NREM) sleep phases 1-3 and REM. NREM is the start of the sleep cycle when the body slowly begins to relax the muscles, body temperature drops and breathing and heart rate becomes more regular. Phases 1 and 2 of NREM last around 25-30 minutes. Phase 3 of NREM is when deep sleep occurs. Deep sleep is the optimal time for regeneration, as anabolic hormones are produced and cells and tissues can be repaired. REM appears at intervals during the night, involving more movement and dreaming. This helps prepare the body to eventually wake up.