While sleep may seem like this expanse of time where our bodies and minds are at complete rest, the opposite is actually true. Sleep typically consists of four stages during which our bodies and minds both slow down and wake up – all while we are fast asleep. We start by going through three stages of non-REM sleep before entering REM (Rapid Eye Movement) sleep and, throughout the course of the night, we go in and out of these stages multiple times.
Stage 1: The Beginning of the Sleep Cycle
This period is a relatively light and short stage of sleep, often considered a transition between wakefulness and sleep. This stage only lasts five to 10 minutes and if you awaken someone during this stage, they might say they were not really sleeping at all.
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.