Sleep Calculator
Find the best times to go to sleep or wake up based on 90-minute sleep cycles. Wake up feeling refreshed by timing your sleep to complete full cycles. Science-backed and free.
😴 How Sleep Cycles Work
Human sleep follows a repeating pattern called the sleep cycle, lasting approximately 90 minutes. Each cycle consists of four stages: N1 (light sleep), N2 (stable sleep), N3 (deep/slow-wave sleep), and REM (Rapid Eye Movement). Adults typically complete 4–6 full cycles per night, totalling 6–9 hours.
The key insight: waking up at the end of a cycle — rather than in the middle of deep sleep — dramatically reduces morning grogginess. This calculator times your sleep and wake windows to align with cycle completion, helping you wake feeling genuinely rested.
📊 Recommended Sleep by Age
14–17 hours
12–15 hours
11–14 hours
9–12 hours
8–10 hours
7–9 hours
7–8 hours
Source: National Sleep Foundation & American Academy of Sleep Medicine guidelines.
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Frequently Asked Questions
❓ What is a sleep cycle?
A sleep cycle lasts about 90 minutes and includes light sleep (N1, N2), deep sleep (N3/slow-wave), and REM sleep. Adults complete 4–6 cycles per night. Waking at the end of a cycle feels much better than waking mid-cycle.
❓ How many hours of sleep do I need?
Adults aged 18–64 need 7–9 hours per night according to CDC and National Sleep Foundation guidelines. Teenagers need 8–10 hours; seniors 7–8 hours. Individual needs vary.
❓ Why do I wake up tired even after 8 hours?
Waking mid-cycle — during deep sleep — causes sleep inertia (grogginess). Timing your alarm to the end of a 90-minute cycle (e.g. 7.5 or 9 hours of sleep) helps you wake during light sleep, feeling more alert.
❓ What is REM sleep?
REM (Rapid Eye Movement) sleep is the dreaming phase, occurring in the second half of each sleep cycle. It is critical for memory consolidation, emotional regulation, creativity, and cognitive performance.
❓ What is sleep debt?
Sleep debt is the cumulative gap between needed and actual sleep. Chronic sleep debt impairs cognition, mood, immune function, and increases risk of diabetes, cardiovascular disease, and obesity.
❓ What time should I go to sleep?
To complete 5 cycles (7.5 hours) and wake at 6 AM: go to bed at 10:14 PM (accounting for ~14 minutes to fall asleep). For 6 cycles (9 hours): go to bed at 8:44 PM. This calculator shows your full set of optimal times.