A sleep cycle lasts approximately 90 minutes and moves back and forth between rapid-eye-movement (REM) and non-rapid-eye-movement (NREM) sleep. The first cycle in the night lasts between 70 and 100 minutes, while later ones last between 90 and 120 minutes.
A healthy adult's sleep cycle typically begins with NREM sleep, which consists of three different stages: stage N1, stage N2, and stage N3.
Stage N1, also known as light sleep, lasts about five minutes and is a transitional stage.
Stage N2, also called true sleep, lasts from 10 to 25 minutes. Your heart rate slows and your body temperature decreases during stage N2.
Stage N3, also referred to as deep sleep or slow-wave sleep, is the deepest stage of sleep and lasts 20 to 40 minutes. It’s difficult to awaken someone from stage N3 sleep because the brain is less responsive to external stimuli. After stage N3, you ascend to lighter NREM sleep stages and then enter REM sleep.
REM sleep, which is also known as active sleep or dream sleep, is characterized by the rapid darting around of the eyes. The REM sleep stage occurs about 70 to 90 minutes after you fall asleep. Dreaming most commonly occurs during REM sleep, but it can also occur during NREM sleep.
Although you may not be conscious of it, your brain is very active during REM. In REM sleep, your breathing is shallow, your heart rate and blood pressure rise, and your arms and legs are temporarily paralyzed.
The first REM sleep episodes in the night may only last one to five minutes, but they become longer as they night progresses.
The reason behind the alternating pattern of NREM and REM sleep is unknown, but scientists speculate that the specific sequences of NREM and REM sleep optimize physical and mental rejuvenation.
Sleep patterns are affected by various factors, including your age, exercise, stress, environmental conditions, and internal clock.
The amount and quality of the sleep you get also has a dramatic effect on your sleep patterns. For example, if you have an irregular sleep schedule or pull an all-nighter, you may have longer and deeper periods of slow-wave sleep.
A normal, healthy adult typically spends 20 to 25 percent of total sleep time in REM sleep, 50 percent in stage N2 sleep, and 30 percent in the remaining stages.
Getting an adequate amount of deep sleep is important because it allows the body to repair itself. Deep sleep is vital to maintaining health, repairing the muscles and tissues, and stimulating growth and development.
REM sleep is also critical because it plays a prominent role in learning and memory. During the REM sleep cycle, your brain processes information you learned in the daytime, replenishes neurotransmitters, and forms neural connections.
There is no set amount of time that everyone needs to sleep each night. The amount varies from person to person, but most adults require at least 7.5 hours to 8 hours of sleep. In simple terms, the amount of sleep you require is what you need to not feel sleepy in the daytime.