Before the 1950’s, scientists used to believe that as people drifted off to sleep, their brains and bodies would go into “shutdown” mode, entering a passive state that allowed them to recover from the previous day. What researchers have since learned: Sleep is a whole lot more complicated, and it’s a much more active state than you might think. In fact, while you’re getting your zzz’s, your brain goes through various patterns of activity. It’s a predictable cycle that includes two distinct parts – NREM, or Non-REM sleep, plus a REM or “Rapid Eye Movement” cycle.
Check out what happens in your body during each phase of sleep:
Stage One: Within minutes (sometimes even within seconds!) of nodding off, your brain produces what are called alpha and theta waves and your eye movements slow down. This introduction to sleep is relatively brief, lasting up to seven minutes. Here, you are in light stage sleep, which means that you’re somewhat alert and can be easily woken. It’s during this stage of sleep that people often indulge in brief “catnaps.”
Stage Two: During this stage, which is also fairly light, the brain produces sudden increases in brain wave frequency known as sleep spindles. Then brain waves slow down. If you were to schedule a “power nap” you’d want to wake up after this stage of sleep.
Stages Three & Four: This stage is the beginning of deep sleep, as the brain begins producing slower delta waves. You won’t experience any eye movement or muscle activity. At this point, it becomes a little harder for you to be awakened, because your body becomes less responsive to outside stimuli. The brain produces even more delta waves and you move into an even deeper, more restorative stage of sleep next. It’s most difficult to wake up during this stage. This is when the body repairs muscles and tissues, stimulates growth and development, boosts immune function, and builds up energy for the next day.
Rapid Eye Movement (REM) Sleep: You generally enter REM sleep about 90 minutes after initially falling asleep, and each REM stage can last up to an hour. An average adult has five to six REM cycles each night. During this final phase of sleep, your brain becomes more active. This is when most dreaming occurs, your eyes jerk quickly in different directions (hence, the name!), heart rate and blood pressure increase, and breathing becomes fast, irregular, and shallow. REM sleep plays an important role in learning and memory function, since this is when your brain consolidates and processes information from the day before so that it can be stored in your long-term memory.
It’s important to note that these phases last for different durations at various ages; an infant’s sleep cycle will look different than that of an adult or elderly individual. On an average night, you move through the stages in a sequential fashion. Most non-REM sleep occurs early in the night and the length of REM periods increases as the night goes on. That’s why there’s a good chance you’ll awaken from a dream in the morning—hopefully, a sweet one!