During sleep, we experience two alternating types or states of sleep over an 8-hour time period: NREM sleep and REM sleep. The cycle repeats itself every 90 minutes.
NREM or Non-Rapid Eye Movement includes 4 stages of sleep. As we shut our eyes for sleep, we first enter light sleep or Stage 1 , the link between being awake and falling asleep.
Stage 2 marks the actual onset of sleep when you become disengaged from the environment, breathing and heart rate are regular and body temperature continues to go down. We spend about ½ of our sleep cycle in the first two stages.
Stage 3 and 4 are the deepest states of sleep when we experience the most restorative sleep, so essential to functioning. Muscles are completely relaxed, blood pressure drops and breathing is slower. During these deep stages of sleep, blood supply to the muscles is increased, energy is restored, tissue growth and repair occur and important hormones are released for growth and development.
REM or Rapid-Eye Movement sleep occurs increasingly over the later part of the night and is also necessary for providing energy to our brains and body. During REM sleep, our brains are active and dreaming occurs.
As we enter REM sleep, our bodies actually become immobile, muscles shut down, and again, we are relaxed. Breathing and heart rate may become irregular. During this stage of active brain activity, the eyes dart back and forth under the eyelids - giving REM sleep its name.
Because getting enough REM sleep may contribute to memory consolidation, it is especially important following a learning experience. REM sleep typically comprises about ¼ of our night.
During a sleep study, electrodes are attached to the head to measure our brain waves at different stages of sleep. This study is called an electroencephelogram or EEG. It measures the brain’s activity by observing brain waves that appear in amplitude, or height, and frequency.
During Stage 1, we see theta, or short, frequent waves, which get higher as we progress through the sleep cycle. During Stage 2, an EEG will reveal more theta waves with a mix of high wave bursts of energy, which are called sleep spindles. As sleep becomes deeper at Stage 3 and 4 and we become more difficult to arouse, more delta or high amplitude, slow-wave sleep appears.
-National Sleep Foundation