😴 Sleep Calculator

Sleep cycles and why timing matters

Sleep progresses through cycles of roughly 90 minutes, moving from light sleep into deep slow-wave sleep and then into REM. Each cycle serves different functions — deep sleep dominates physical restoration, REM is associated with memory consolidation and emotional processing.

Waking at the end of a cycle, in light sleep, generally feels easier than being pulled out of deep sleep mid-cycle. That is the reasoning behind planning bedtime or wake time in 90-minute multiples, which this calculator does after allowing about 15 minutes to fall asleep.

The 90-minute figure is an average. Individual cycles range from roughly 70 to 120 minutes and vary across the night, with deep sleep concentrated early and REM periods lengthening toward morning. Treat the suggested times as useful approximations, not precision instruments.

How much sleep is enough

The proportion of people who genuinely function well on less than six hours is very small — far smaller than the number who believe they belong to it. Chronic short sleep degrades reaction time and judgement in ways that are difficult to notice from the inside, because the ability to assess your own impairment degrades along with everything else.

What actually improves sleep

Consistency is the strongest lever. A regular wake time, including at weekends, stabilises the circadian rhythm more effectively than any other single change.

Morning light anchors that rhythm. Fifteen to thirty minutes of daylight shortly after waking is more effective than evening screen restriction for most people.

Caffeine has a half-life of roughly five to six hours, so an afternoon coffee is still measurably present at bedtime. Alcohol is worse than it feels: it shortens time to sleep onset while suppressing REM and fragmenting the second half of the night.

A cool, dark room supports the drop in core body temperature that accompanies sleep onset. Around 18°C is commonly recommended.

Common questions

Can I catch up on lost sleep at the weekend?

Partially. Some recovery occurs, but shifting wake times by hours creates a jet-lag-like effect that makes Monday harder.

Are naps helpful?

Short naps of 10 to 20 minutes can improve alertness without grogginess. Longer naps risk waking from deep sleep and may interfere with that night's sleep.

Why do I wake before my alarm?

With a consistent schedule the body anticipates waking and begins raising cortisol beforehand. It is generally a sign the rhythm is well established.