The sleep cycle is the repeating sequence of sleep stages that a person progresses through during a night’s sleep. Each cycle lasts approximately 90 minutes and consists of a progression through the non-REM sleep stages (N1, N2, and N3) followed by a period of REM sleep. Over the course of a typical night, most adults complete four to six sleep cycles. The structure of each sleep cycle changes across the night. In the first half of the night, sleep cycles are dominated by deep, slow-wave sleep (stage N3), which is the most physically restorative phase. As the night progresses, deep sleep decreases and the proportion of REM sleep in each cycle increases. By the early morning hours, sleep cycles consist mostly of lighter N2 sleep and progressively longer REM periods, which is when the most vivid dreaming occurs. Understanding the sleep cycle helps explain several common sleep experiences. Waking naturally at the end of a sleep cycle — when sleep is lightest — tends to feel more refreshing than being abruptly awakened from deep sleep, which causes the grogginess known as sleep inertia. This is why some people find that waking naturally, without an alarm, feels more restorative even if the total sleep duration is similar. The sleep cycle also explains why nap timing and duration matter. A short nap of 10 to 20 minutes avoids entry into deep sleep, preventing sleep inertia and not affecting nighttime sleep. A longer nap of around 90 minutes may include a full sleep cycle, including REM sleep, and can be more restorative for creative thinking and emotional processing. Disruptions to the sleep cycle — from sleep disorders, alcohol, stress, or frequent awakenings — reduce the restorative value of sleep and impair physical and cognitive functioning.