A night of sleep has a typical structure that includes rapid eye movement and non-rapid eye movement. Falling asleep is a gradual progression of stages. Body and brain activity change in each of the four stages.
In stage one, the brain waves and muscle activity slows down. Stage two contains about half of the sleep period. There are calmer brain waves and no eye movement. Breathing slows, and the body’s temperature drops slightly. A person in stage two can easily be awoken. In stages three and four, it becomes harder to wake up someone. Breathing slows into a rhythm, blood pressure and body temperature drop, and the muscles relax. Stages three and four are known as slow-wave sleep because during these stages are the slowest of all brain waves.
Rapid eye movement begins after about 90 minutes of sleep. It brings the slow-wave sleep into a state of being close to awake. Brain waves speed up to the same or faster as when a person is awake. Most dreams happen during this stage. The heart beats quickly, blood pressure rises, eyes move, and breathing becomes rapid. The body does not move. Rapid eye movement can last from five to 30 minutes. Non-rapid eye movement begins again after rapid eye movement. This cycle repeats four to six times every night.