Sleep happens in cycles of roughly 90 minutes. Each cycle moves through light sleep, deep sleep, and REM sleep before repeating. Waking at the end of a cycle — rather than in the middle of one — is what makes the difference between feeling rested and feeling groggy. Cycles vary from person to person, typically between 70 and 110 minutes — adjust the slider below if you know your own pattern.
Sleep cycle science comes from polysomnography research — the measurement of brain activity during sleep. A full night typically consists of four to six cycles, each lasting around 90 minutes, though this varies by age, individual biology, and how tired you are. The first cycles of the night contain more deep (slow-wave) sleep; later cycles contain more REM sleep, which is associated with dreaming and memory consolidation. Waking mid-cycle — when you are deepest into slow-wave or REM sleep — is what causes sleep inertia, the groggy, disoriented feeling that can last for an hour or more. The fall-asleep offset accounts for sleep onset latency: the time between lying down and actually falling asleep, which averages around 14 minutes in healthy adults but varies considerably.