Reading the clock
Reading the clock
Why cycles beat hours
Sleep runs in cycles of roughly 90 minutes, from light sleep into deep sleep and REM and back again. Wake mid cycle, especially from deep sleep, and you get that heavy, groggy feeling no coffee fixes. Time the night as whole cycles and the alarm meets you in light sleep, which is why 7.5 hours can feel better than 8.
Real cycles vary from person to person, roughly 70 to 120 minutes. Treat these times as a smart starting point, then adjust to how you wake
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How long is a sleep cycle?
About 90 minutes on average, moving from light sleep into deep sleep and then REM before starting again. Real cycles vary from person to person and through the night, roughly 70 to 120 minutes, which is why these times are a smart starting point rather than a precise science.
How much sleep do I need?
The NHS says most adults need 7 to 9 hours a night, which is why the 5 and 6 cycle options are highlighted. Teenagers need more, around 8 to 10 hours, and children more again. If you regularly feel fine on less, you're rare, most people underestimate what tiredness is costing them.
Why do I wake up groggy even after a long sleep?
Usually because the alarm landed in the middle of a cycle, especially deep sleep. That heavy, disoriented feeling is called sleep inertia. Waking at the end of a cycle, in light sleep, is the whole idea behind timing your night in full cycles.
How long does it take to fall asleep?
About 10 to 20 minutes is typical, which is why these times include a 15 minute buffer. Regularly dropping off the moment your head hits the pillow can actually be a sign you're short on sleep, and lying awake much past 30 minutes most nights is worth looking at.
Is 6 hours of sleep enough?
For most adults, no. It's below the NHS 7 to 9 hour guidance, and running on 6 hours night after night builds up a debt that shows in mood, focus and appetite. The 4 cycle option is there for the nights you're squeezed, not as a target.
When should I see a GP about sleep?
If you've had trouble sleeping for more than a few weeks, if you snore loudly with gasping or choking sounds, or if you're exhausted in the day no matter how long you spend in bed. Those are worth a GP conversation rather than another calculator.
Why can I trust this calculator?
The assumptions are stated right on the page, roughly 90 minute cycles, about 15 minutes to fall asleep and the NHS 7 to 9 hour adult range, and every suggested time is simple arithmetic on top of them that you can check yourself. Nothing you type is saved, and there are no ads or mattress links paying for a particular answer.
This calculator is for general information only. It isn't medical advice, a diagnosis or a treatment plan, and Biofaze isn't a healthcare provider or a medical device. Speak to your GP or another qualified health professional before acting on these numbers, especially if you're pregnant, under 18 or managing a health condition.
Sleep need figures come from NHS guidance, most adults 7 to 9 hours, and the roughly 90 minute cycle length from standard sleep research. Guidance last checked July 2026.