What time should you actually go to bed?

Why 90-minute sleep cycles?

A full sleep cycle — moving through light sleep, deep sleep, and REM sleep — averages about 90 minutes, though it varies somewhat by individual and by night. Waking up in the middle of a cycle, particularly during deep sleep, tends to produce more grogginess than waking up between cycles, which is the idea behind timing sleep or wake-up around cycle boundaries rather than an arbitrary duration.

Why is there a 15-minute buffer added?

Most people don't fall asleep the instant they lie down — the buffer accounts for typical time-to-fall-asleep (sleep latency), so the cycle count lines up with when you're actually asleep, not just when you got into bed. Adjust it above if you know your own latency tends to be longer or shorter.

Is this based on real sleep tracking, or a fixed assumption?

It's a fixed 90-minute average, not a measurement of your actual sleep architecture — real cycle length varies by person, by age, and night to night, and only a sleep study or a consumer sleep tracker analyzing your actual data can measure yours specifically. This calculator is a reasonable general planning heuristic, not a substitute for that kind of measurement.

Worked example: waking up at 7:00 AM with a 15-minute fall-asleep buffer: going to bed at 11:15 PM gives 5 full cycles (7.5 hours of sleep), while 9:45 PM gives 6 full cycles (9 hours).

Why 90 minutes

Sleep runs in cycles that progress from light sleep through deep slow-wave sleep into REM, then back. A full cycle averages about 90 minutes, and most people complete five or six in a night — which is where the familiar 7.5 to 9 hour range comes from.

The point of timing by cycles is that where you wake matters as much as how long you slept. Waking at the end of a cycle, in light sleep, feels straightforward. Waking mid-cycle out of deep sleep produces sleep inertia — the heavy, disoriented feeling that can persist for half an hour or more. Six full cycles often feels better than an awkwardly-timed seven hours.

The 90 minutes is an average

Real cycles range from roughly 70 to 120 minutes and vary between people and across the night. Early cycles carry more deep sleep; later ones carry proportionally more REM, which is why the last stretch before waking is when most dreaming happens — and why cutting sleep short costs you disproportionately much REM.

Add about 15 minutes of sleep latency — the time to actually fall asleep — when working backwards from a wake time. Counting from when you get into bed rather than when you fall asleep is the usual reason the arithmetic doesn't work out.

Consistency beats optimisation

A regular schedule matters more than hitting exact cycle boundaries. Your circadian rhythm entrains to consistent timing, and waking at the same time daily — including weekends — does more for how you feel than any calculated bedtime.

"Social jetlag", the habit of shifting sleep two or more hours later at weekends, produces something close to the effect of flying across time zones every week.

What actually helps

• Morning daylight — the strongest signal for setting your body clock.
• Caffeine has a 5-6 hour half-life. A 3pm coffee leaves a meaningful dose at bedtime.
• Alcohol shortens sleep latency and wrecks sleep quality, suppressing REM and fragmenting the second half of the night.
• A cool, dark room. Core temperature needs to fall for sleep to begin.
• Naps of 20 minutes or a full 90 — anything between risks waking from deep sleep.

Persistent trouble sleeping despite good habits is worth raising with a doctor; insomnia and sleep apnoea are common, treatable, and not fixed by better bedtime arithmetic.