Sleep cycles, and why ninety minutes is only an average

Through the night, sleep moves through a repeating pattern: lighter sleep, deeper sleep, and then REM sleep, the stage most associated with vivid dreaming. One pass through that pattern is a sleep cycle.

How long is a cycle?

On average, around ninety minutes. But the real figure varies from person to person and even across a single night. Early cycles tend to have more deep sleep; later ones have more REM.

Ninety minutes is an average, not a rule.

Why people plan around cycles

Waking from deep sleep can leave you groggy for a while. Waking at the lighter end of a cycle often feels easier. So some people like to count back from their alarm in cycles to pick a bedtime.

How to use it sensibly

• Treat the result as a starting point and adjust it over a week or two.
• Allow time to fall asleep — usually somewhere between ten and twenty minutes.
• A regular wake-up time, kept even at weekends, matters more than exact arithmetic.

If you regularly wake exhausted however long you sleep, that is worth raising with a doctor.