How the Sleep Calculator works
You sleep in repeating cycles of about 90 minutes, moving from light sleep into deep sleep and dreaming, then back again. Waking up in the middle of a deep cycle is what leaves you groggy — even after eight hours. Waking at the end of a cycle, when sleep is lightest, is what makes you feel refreshed. This calculator counts back (or forward) in 90-minute cycles and adds the ~15 minutes it takes an average person to fall asleep, so you can aim for a natural, gentle wake-up.
These are guides, not medical advice. Most adults do best on 5–6 full cycles (7.5–9 hours). If you’re often tired despite enough sleep, it’s worth speaking to a doctor.
Frequently Asked Questions
How many hours of sleep do I actually need?
Most adults need 7–9 hours — that’s 5 to 6 complete 90-minute cycles. The calculator highlights the 5- and 6-cycle options as the sweet spot for feeling rested.
Why do I wake up tired after 8 hours?
Because your alarm likely went off in the middle of deep sleep. Aligning your wake-up to the end of a cycle — even if that means slightly less total sleep — often leaves you feeling more refreshed.
Why add 15 minutes?
It takes the average person about 15 minutes to actually fall asleep after getting into bed, so the calculator builds that in when working out your times.
What if I can’t fall asleep at these times?
Use them as a target and build a calming wind-down routine. Our sleep guides cover simple habits — and our best-sleep-aids picks — that help you drift off closer to your ideal bedtime.