A sleep calculator determines the ideal bedtimes to wake up at a specific time feeling rested, based on 90-minute sleep cycles.
About this calculator
A sleep calculator determines the ideal bedtimes to wake up at a specific time feeling rested, based on 90-minute sleep cycles. Sleep occurs in cycles of approximately 90 minutes each, and waking at the end of a cycle (rather than in the middle) results in feeling more refreshed and alert.
Most adults need 5–6 complete sleep cycles (7.5–9 hours). Adding 15 minutes for average sleep onset time, the calculator shows bedtimes aligned to cycle endings.
Common uses
- Find the ideal bedtime to wake up at 6:00 AM feeling refreshed
- Plan a nap timing to avoid waking mid-cycle (sleep inertia)
- Optimise sleep schedule for shift workers
- Identify the bedtime for exactly 7.5 or 9 hours of sleep
Frequently asked questions
What are sleep cycles and why do they matter?
A sleep cycle is approximately 90 minutes long and consists of light sleep, deep sleep, and REM sleep stages. Waking during deep sleep or REM causes grogginess (sleep inertia). Waking at the end of a cycle, when you are in lighter sleep, makes waking easier and more refreshing.