Sleep Calculator
Sleep runs in roughly 90-minute cycles, and waking at a cycle's edge beats waking inside one — even with fewer total minutes. Set your alarm time and read the bedtimes that land on whole cycles, or set a bedtime and read the wake-ups. The average 15 minutes of falling asleep is already counted.
Architecture, not just duration
A night is a staircase walked four to six times: light sleep, deep slow-wave sleep, REM, and briefly almost-awake — that is one cycle. Deep sleep dominates the early cycles and repairs the body; REM dominates the late ones and consolidates memory. Cutting the night short does not trim evenly — it amputates the REM-rich end.
That is why the table thinks in whole cycles rather than round hours: eight fashionable hours that plant the alarm mid-cycle can lose to seven and a half that end on an edge.
Frequently asked questions
Why does waking mid-cycle feel so bad?
Because the middle of a cycle is deep slow-wave sleep, and an alarm there produces sleep inertia — the leaden, cotton-headed hour that no coffee fixes. The end of a cycle passes through light sleep, where the same alarm lands gently. Same night, different minute, different morning.
How many cycles do I actually need?
Five to six for most adults — 7.5 to 9 hours — which is why those two rows carry the mark. Four is a survivable deficit, three is the floor this page will even print. Chronically choosing the short rows is not optimisation; it is debt with compounding interest.
Is everyone's cycle exactly 90 minutes?
No — real cycles run roughly 80 to 110 minutes and lengthen through the night. Ninety is the honest average and the planning number. If the calculator's best times consistently feel wrong by half an hour, your personal cycle differs: shift bedtime in 15-minute steps and keep what works.
What about the 15 minutes of falling asleep?
Sleep latency — the gap between lights out and sleep — averages 10 to 20 minutes in healthy sleepers, so the tool books 15. Regularly crashing in under five minutes is itself a sign of sleep deprivation, and lying awake past thirty suggests moving bedtime later, not earlier.
Related tools
- BMI CalculatorBody mass index on the WHO adult scale, metric or imperial, with the normal-range weight for your height and the scale's limits stated plainly.
- Calorie Calculator (BMR & TDEE)Your basal metabolism by Mifflin–St Jeor and your daily calories by activity level, with honest ±500 kcal bands for losing or gaining.
- Ideal Weight CalculatorIdeal body weight by Devine, Robinson, Miller and Hamwi — the formulas medicine actually cites — plus the healthy-BMI weight band for your height.