Sleep Calculator
Wake up refreshed by timing your sleep in 90-minute cycles
Go to bed at one of these times
Wake up at one of these times
How to Use
- Pick a mode — Choose “I want to wake at…” to get suggested bedtimes for a chosen wake-up time, or “I want to sleep now” to get suggested wake-up times starting from the current moment.
- Set the details — Enter your wake-up time (wake mode) and adjust the fall-asleep time if you usually need more or less than the default 15 minutes to drift off.
- Read the suggestions — Each card shows a time, the number of 90-minute cycles, and the total sleep duration. Recommended options (5–6 cycles) are highlighted in blue.
- Switch clock format — Use the 12h/24h toggle to display times in your preferred format.
Features
- Two calculation modes — Plan bedtimes backward from a wake-up time, or wake-up times forward from right now.
- 90-minute cycle math — Suggestions are spaced exactly one sleep cycle apart, so you wake between cycles instead of during deep sleep.
- Configurable latency — Accounts for the minutes you need to fall asleep (default 15).
- Recommended highlight — The 5–6 cycle options (7.5–9 hours) are marked as best for most adults.
- 12h/24h display — Defaults follow your locale and can be toggled manually.
- 100% client-side — No data leaves your browser.
Use Cases
- Workday planning: Find the latest bedtime that still gives you 5–6 full cycles before your alarm.
- Late nights: Going to bed right now? See which wake-up times land at the end of a cycle.
- Nap and shift scheduling: Time shorter sleeps around cycle boundaries to reduce grogginess.
Related Tools
- Countdown Timer — count down to your wake-up time or any event.
- BMI Calculator — another health tool for tracking body metrics.
- World Clock — compare times across time zones when traveling.
Frequently Asked Questions
Why 90-minute sleep cycles?
Sleep moves through repeating cycles of light sleep, deep sleep, and REM sleep, each lasting roughly 90 minutes. Waking at the end of a cycle, during light sleep, feels noticeably easier than being woken mid-cycle from deep sleep.
What does the fall-asleep time setting do?
Most people need about 15 minutes to fall asleep after going to bed. The calculator shifts every suggestion by this latency so the cycles are counted from when you actually fall asleep, not from when you lie down. Adjust it if you typically fall asleep faster or slower.
Which option should I pick?
The recommended options (5–6 cycles, 7.5–9 hours of sleep) are highlighted because most adults do best in that range. Four cycles (6 hours) is shown as a minimum for occasional short nights, but it should not become a habit.
Does the calculator store any of my data?
No. All calculations run entirely in your browser — no wake times, bedtimes, or settings ever leave your device.