Sleep happens in roughly 90-minute cycles, and waking mid-cycle is why you sometimes feel groggy after "enough" hours. This calculator counts backwards in complete cycles — including the average 14 minutes it takes to fall asleep — so you wake at the lightest point of your sleep.
About this tool
Good to know: Most adults need 5–6 complete cycles (7.5–9 hours). Consistency matters more than perfection — a steady bedtime trains your body clock better than occasional catch-up sleep. Persistent daytime sleepiness despite enough hours is worth discussing with a doctor.
This tool is for general information and screening purposes only. It is not medical advice and cannot diagnose any condition. Always consult a qualified healthcare professional about your health.