Sleep Calculator

Sleeping in cycles

Sleep progresses through cycles of roughly 90 minutes, each moving from light sleep into deep slow-wave sleep and then into REM sleep before returning towards lighter stages.

Waking at the end of a cycle, in light sleep, generally feels better than waking mid-cycle from deep sleep, when sleep inertia is most pronounced.

Which is the basis for cycle-based bedtime calculators: count backwards in 90 minute blocks from the required waking time, adding around 15 minutes to fall asleep.

The 90 minute figure is an average

Cycle length varies between individuals from roughly 70 to 120 minutes, and it varies within a night. Early cycles contain more deep sleep and later ones more REM.

So a calculation assuming exactly 90 minutes will be wrong for many people and will drift over the course of a night for everyone.

The practical consequence is that these calculations are approximate. They are a reasonable way to choose between two candidate bedtimes and a poor basis for treating one specific minute as correct.

Duration matters more than timing

Adults generally need seven to nine hours, and both shorter and considerably longer durations are associated with worse health outcomes in large cohort studies.

Teenagers need eight to ten and have a biologically later chronotype, which is why early school start times are an ongoing subject of research and argument.

Optimising the wake point within a cycle while sleeping six hours is optimising the wrong variable. Total duration is the thing with the evidence behind it, and cycle timing is a refinement on top of an adequate amount.

What actually improves sleep

Consistency of timing is the strongest single lever, including at weekends. Social jetlag, the shift between weekday and weekend sleep timing, is associated with metabolic and mood effects independent of duration.

Light exposure regulates the circadian system: bright light in the morning advances the clock, and light in the evening delays it. The evening effect is why screen use before bed matters, though the size of that effect is more modest than commonly claimed.

Caffeine has a half-life of around five hours and considerable individual variation, so an afternoon coffee is still meaningfully present at bedtime for many people. Alcohol induces sleep and fragments its second half, reducing REM, which is why sleep after drinking is unrefreshing.

When to get help

Persistent difficulty falling or staying asleep, for three or more nights a week over three months, meets the usual definition of chronic insomnia and is treatable.

Cognitive behavioural therapy for insomnia is the recommended first-line treatment in most guidelines and outperforms medication for long-term outcomes. It is available through the NHS in the UK and through several digital programmes.

Loud snoring with pauses in breathing, choking, and daytime sleepiness despite adequate time in bed suggests obstructive sleep apnoea, which is common, underdiagnosed and associated with serious cardiovascular risk. That warrants a medical assessment rather than any adjustment to bedtime.

There is also a distinction worth drawing between sleep timing and chronotype. Some people are genuinely biologically inclined towards later sleep and waking, and that inclination is partly genetic and largely resistant to willpower. Someone whose natural window is one in the morning to nine will not become an early riser through discipline, and forcing a schedule that fights their chronotype produces chronic short sleep rather than an adjusted clock. Where the work schedule allows any flexibility, matching it to the chronotype is more effective than any bedtime calculation.