How many sleep cycles you get and when to wake. A cycle is roughly 90 minutes.
Sleep runs in repeating cycles that move through light sleep, deep sleep and REM. One cycle in an adult lasts roughly 90 minutes, so a night usually holds four to six of them. This calculator picks times so that the alarm falls between cycles rather than inside one.
Because timing matters as much as duration. Waking in the middle of deep sleep produces the grogginess known as sleep inertia, which can last half an hour. Twenty minutes of extra sleep that lands inside a deep phase can therefore feel worse than slightly less sleep.
Most adults need seven to nine hours, which is roughly five or six cycles. The requirement is individual and changes with age — teenagers and children need considerably more. Counting cycles helps you choose a time, but it does not substitute for enough total sleep.
About fifteen minutes on average, which this calculator allows for when working out a bedtime. If falling asleep regularly takes more than half an hour, or happens almost instantly, that is itself informative: the first often points to insufficient tiredness or stress, the second to sleep deprivation.