Sleep Calculator
Find the best times to wake up or go to bed based on natural 90-minute sleep cycles. Waking at the end of a cycle helps you feel refreshed instead of groggy.
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Frequently Asked Questions
Why are sleep cycles 90 minutes?
A complete cycle includes NREM Stage 1 (light), Stage 2 (light with sleep spindles), Stage 3 (deep slow-wave), and REM sleep. Each runs approximately 90 minutes. Waking at the end of a cycle — rather than mid-cycle during deep sleep — dramatically reduces grogginess.
How many hours of sleep do adults need?
Most adults need 7–9 hours (5–6 complete 90-minute cycles). Six hours is the functional minimum for most but leads to measurable cognitive decline long-term. 7.5 hours (5 cycles) is optimal for the majority of adults.
What is sleep debt?
Sleep debt is cumulative: it builds when you regularly sleep less than needed. Some short-term recovery is possible with extra sleep, but chronic deprivation has lasting effects. The best strategy is consistent sleep and wake times 7 days a week.