Sleep Calculator
Find the best time to wake up or go to sleep based on 90-minute sleep cycles — so you wake feeling refreshed, not groggy.
How sleep cycles work
Sleep is not a single uniform state — it progresses through distinct stages in roughly 90-minute cycles. Each cycle moves through light sleep (N1, N2), deep slow-wave sleep (N3), and REM sleep. Waking mid-cycle, especially during deep sleep, produces sleep inertia: the heavy, disoriented feeling that can last 20–30 minutes regardless of how long you slept.
By aligning your alarm to the end of a complete cycle, you wake during light sleep when arousal is easiest. Most adults need 5–6 cycles (7.5–9 hours) to feel fully rested. Athletes and those in heavy training phases benefit from 6 cycles to maximise the growth hormone secretion and muscle repair that occurs during deep sleep.
The calculator adds 14 minutes to the bedtime — the average time it takes to fall asleep. If you typically fall asleep faster or slower, adjust your target time accordingly.
Frequently Asked Questions
- Why are sleep cycles 90 minutes?
- A complete sleep cycle progresses through light sleep, deep (slow-wave) sleep, and REM sleep over approximately 90 minutes. This duration is consistent across most adults, though it can range from 80–120 minutes. Waking mid-cycle — especially during deep sleep — triggers sleep inertia, the groggy, disoriented feeling that can persist for up to 30 minutes.
- How many sleep cycles do I need?
- Most adults function best on 5–6 complete cycles (7.5–9 hours). Athletes and those doing heavy training often benefit from 6 cycles. Four cycles (6 hours) is a minimum that most people can sustain short-term without significant cognitive impairment, but it's not optimal for muscle recovery or hormonal health.
- Why does the calculator add 14 minutes?
- Research shows the average time to fall asleep (sleep latency) is around 10–20 minutes. The calculator uses 14 minutes as a reasonable middle estimate. If you fall asleep faster or slower, adjust your bedtime manually.
- Does sleep timing really affect recovery?
- Yes. The majority of growth hormone release — critical for muscle repair — occurs during deep slow-wave sleep in the first half of the night. Cutting sleep short disproportionately reduces REM sleep in the later cycles, impairing memory consolidation and cognitive recovery. Consistent timing also stabilises your circadian rhythm, improving sleep quality over time.