NREM Stage 1: Light Sleep
This is the lightest stage of NREM sleep. Often defined by the presence of slow eye movements, this drowsy sleep stage can be easily disrupted causing awakenings or arousals. Muscle tone throughout the body relaxes and brain wave activity begins to slow from that of wake. Occasionally people may experience hypnic jerks or abrupt muscle spasms and may even experience sensation of falling while drifting in and out of Stage 1.
NREM Stage 2: Deeper Sleep
This the first actual stage of defined NREM sleep. Awakenings or arousals do not occur as easily as in Stage 1 sleep and the slow moving eye rolls discontinue. Brain waves continue to slow with specific bursts of rapid activity known as sleep spindles intermixed with sleep structures known as K complexes. Both sleep spindles and K complexes are thought to serve as protection for the brain from awakening from sleep. Body temperature begins to decrease and heart rate begins to slow.
NREM Stage 3: Deepest Sleep
This known as deep NREM sleep. The most restorative stage of sleep, stage 3 consists of delta waves or slow waves. Awakenings or arousals are rare and often it is difficult to awaken someone in Stage 3 sleep. Parasomnias (sleepwalking, sleep talking, and night terrors) occur during the deepest stage of sleep.
Stage 4: Rapid Eye Movement (REM)
Most commonly known as the dreaming stage as this is the stage where dreams occur. Eye movements are rapid, moving from side to side and brain waves are more active than in Stages 2 & 3 of sleep. Awakenings and arousals can occur more easily in REM; being woken during a REM period can leave one feeling groggy or overly sleepy.