Sleep Calculator
Find the best time to go to sleep or wake up based on 90‑minute sleep cycles, so you wake feeling rested instead of groggy.
| Sleep cycles | Sleep duration | Bedtime |
|---|---|---|
| 6 cycles | 9 h | 9:45 PM |
| 5 cycles | 7.5 h | 11:15 PM |
| 4 cycles | 6 h | 12:45 AM |
Based on 90‑minute sleep cycles plus ~15 minutes to fall asleep. Waking at the end of a cycle helps you feel more rested. Most adults need 7–9 hours.
How to use this calculator
Choose whether you're planning around a wake-up time or a bedtime, then enter that time. The calculator suggests times that complete whole sleep cycles, with the best option giving a full 9 hours.
How sleep cycles work
During the night you move through cycles of light sleep, deep sleep, and REM, each lasting about 90 minutes. Completing full cycles — and waking at the end of one — generally leaves you feeling more refreshed than waking mid‑cycle.
Worked example
To wake at 7:00 AM with six full cycles, you'd aim to be asleep by about 9:45 PM (allowing ~15 minutes to drift off).
A general guide, not medical advice. See a professional about ongoing sleep problems.
Frequently asked questions
How does a sleep calculator work?
Sleep happens in roughly 90‑minute cycles. The calculator counts back (or forward) in full cycles from your wake or bedtime, adding about 15 minutes to fall asleep, so you wake at the end of a cycle rather than mid‑cycle.
Why wake at the end of a sleep cycle?
Waking during deep sleep tends to leave you groggy, while waking at the lighter end of a cycle usually feels more refreshed. Timing sleep in whole cycles aims for the latter.
How many hours of sleep do I need?
Most adults need 7–9 hours, which is about five to six 90‑minute cycles. Teenagers and children generally need more.
Is the 90‑minute cycle exact?
It's an average — real cycles range from about 70 to 120 minutes and vary by person and night. Use the suggested times as helpful targets, not strict rules.