Free tool
Sleep calculator
We sleep in roughly 90-minute cycles, and waking at the end of one — instead of the middle — is what makes the difference between groggy and refreshed. Pick a time and this finds the sweet spots.
Green times are the 7.5–9 hour sweet spot (5–6 cycles). Add ~15 minutes to fall asleep — already included.
Sleep is your highest-leverage number
Your Body Data, Explained shows how sleep drives your HRV, recovery, and energy — and how to read the scores without obsessing.
How the sleep calculator works
A full night of sleep is usually 5–6 cycles of about 90 minutes each. This tool counts backward (or forward) in 90-minute blocks and adds roughly 15 minutes to fall asleep, so your alarm lands near the end of a cycle — where waking feels easiest.
How many hours of sleep do I need?
Most adults do best on 7–9 hours (5–6 cycles). Consistency matters as much as the total — same bedtime and wake time, even at weekends.
Why do I wake groggy after 8 hours?
Because the alarm caught you mid-deep-sleep. That grogginess (sleep inertia) is about timing, not just total hours — which is exactly what this tool helps with.