Health Hub · Free tool
Sleep Calculator
Work out the best time to go to bed using 90-minute sleep cycles — then add up your week's sleep debt and get a realistic plan to repay it.
What time should I go to bed?
Times include ~15 minutes to fall asleep. Waking at the end of a 90-minute cycle feels far better than being dragged out of the middle of one — which is why 7½ hours often beats 8.
How big is my sleep debt?
How much sleep do I need?
Most adults need 7–9 hours a night — the genuine short-sleepers who thrive on less are far rarer than the people who've simply stopped noticing how tired they are. Needs shift across life:
| Age | Sleep needed (per 24h) |
|---|---|
| Teenagers (14–17) | 8–10 hours |
| Adults (18–64) | 7–9 hours |
| Over 65 | 7–8 hours, often lighter and earlier |
The honest self-test: if you need an alarm every day, fall asleep within five minutes of lying down, or sleep two hours longer given the chance at weekends — you're under-sleeping.
Why 90-minute sleep cycles matter
Sleep runs in cycles of roughly 90 minutes, moving from light sleep through deep sleep into REM. Wake at a cycle's end and you surface from light sleep feeling human; wake mid-cycle — especially out of deep sleep — and you get that concrete-headed feeling no coffee fixes. That's why the calculator counts backwards from your alarm in 90-minute steps, plus about fifteen minutes to drop off. It's also why a planned 7½ hours can genuinely beat a mistimed 8.
Sleep debt: what it is and what it does
Sleep debt is the running gap between what your body needs and what it gets — and it compounds. An hour a night sounds survivable, but by Friday it performs like a five-hour all-nighter spread thin: slower reactions, worse mood, stronger cravings, weaker immunity. The research is blunt — after a fortnight of six-hour nights, people perform like they've been awake for 24 hours straight while rating themselves as fine. The debt hides from its owner.
Can you repay sleep debt?
Partially, and only gradually — one long Sunday lie-in doesn't clear a week's arrears, and lying in past mid-morning pushes your body clock later, making Monday worse. What works: go to bed 30–60 minutes earlier (rather than waking later) for a week or two, keep your wake time fixed, and use a 20-minute early-afternoon nap if you need a bridge. Our guides to falling asleep faster and 3am waking cover the execution, and if a rough patch needs breaking, the honest options are in our sleeping tablets guide.
Frequently asked questions
Is 6 hours of sleep enough?
For a night, you'll cope. As a lifestyle, almost certainly not — six-hour sleepers accumulate measurable impairment while adapting to how it feels, which is the trap.
Are naps good or bad?
A 10–20 minute nap before ~3pm is a genuine performance tool. Longer or later naps eat into night-time sleep pressure and can start a vicious cycle.
Why do I wake up groggy even after a long sleep?
Usually mid-cycle waking — try shifting your bedtime so your total is a multiple of 90 minutes. If long sleep plus grogginess is constant, mention snoring and apnoea to your GP.