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Sleep Calculator Guide
Sleep runs in cycles of roughly 90 minutes. Waking at the end of a cycle — rather than in the middle of deep sleep — tends to feel much better, even on the same total hours.
Bedtime = wake time − (cycles × 90 minutes) − time to fall asleep
To wake at 07:00 after 5 cycles, allowing 15 minutes to drop off, go to bed at 23:15.
Why Cycle Timing Beats Total Hours
Six hours ending cleanly at the end of a cycle can leave you sharper than seven hours that ends mid-way through deep sleep. Being woken from slow-wave sleep produces sleep inertia — that thick, disoriented feeling that can hang around for half an hour.
This is why the alarm feels brutal some mornings and fine on others despite similar hours.
90 Minutes Is an Average, Not Your Number
Real cycles range from about 70 to 120 minutes, they vary between people, and they vary within a night — early cycles carry more deep sleep, later ones more REM. Any calculator using a fixed 90 is giving you a decent approximation, not a precision instrument.
Treat the times as a shortlist. If 23:15 leaves you groggy, try the next option up. Your own experience beats the arithmetic.
How Many Cycles
Most adults need 7–9 hours, which is 5–6 cycles. Four cycles (6 hours) works short-term for some people and is a debt for most. Chasing cycle timing while sleeping five hours a night is optimising the wrong variable.
The Latency Number Matters
Healthy sleep onset is 10–20 minutes. Falling asleep the moment your head lands usually signals you're under-slept, not that you're a great sleeper. Consistently over 30 minutes — or lying awake in the small hours — is worth raising with a doctor rather than solving with a calculator.
What Actually Helps
A consistent wake time is the single most useful lever, and it matters more than bedtime. Your body clock anchors to when light hits your eyes, so getting up at the same time — weekends included — does more than any bedtime routine.
Beyond that: cool, dark room; no caffeine after early afternoon (it has a half-life of about five hours, so a 3pm coffee is still half-present at 8pm); and alcohol will help you fall asleep while wrecking the second half of the night.
Related: heart rate zone calculator, hours calculator.
This is an estimate for general information, not medical advice. Your needs depend on factors no calculator can see. Speak to a doctor or a registered dietitian before making significant changes, especially if you have a health condition or a history of disordered eating.
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