😴 Health Tool

Sleep Calculator

Wake up refreshed, not groggy. Enter when you need to wake up — get the bedtimes that land you at the end of a 90-minute sleep cycle.

😴 Sleep Cycle Planner

Sleep happens in ~90-minute cycles. Waking at the end of a cycle (not the middle) helps you feel refreshed. 5–6 cycles = 7.5–9 hours is ideal for most adults.

Best bedtimes
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6 cycles · 9 hours

The 90-Minute Sleep Cycle

Through the night you cycle between light sleep, deep (slow-wave) sleep and REM sleep, with each full cycle lasting about 90 minutes. Alarms that go off in the middle of deep sleep cause 'sleep inertia' — that groggy, heavy feeling. Waking near the end of a cycle, in lighter sleep, feels much easier.

Wake at 6:30 AM → ideal bedtimes: 9:15 PM, 10:45 PM or 12:15 AM (each is a whole number of cycles plus 15 min to fall asleep).

Tips for Better Sleep

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❓ Frequently Asked Questions
How does the sleep cycle calculator work?›
Sleep runs in roughly 90-minute cycles (light → deep → REM). Waking at the end of a cycle feels far more refreshing than waking mid-cycle. This tool counts back (or forward) in 90-minute blocks from your chosen time, and adds the ~15 minutes most people take to fall asleep.
How many hours of sleep do I need?›
Adults generally need 7–9 hours, which is 5–6 complete cycles. Teenagers need 8–10 hours, and children more. The 6-cycle (9 hour) and 5-cycle (7.5 hour) options are highlighted as recommended; 3–4 cycles are 'survival' amounts for an unavoidably short night.
What time should I sleep to wake up at 6 AM?›
To wake at 6:00 AM with a 15-minute fall-asleep buffer: about 8:45 PM (6 cycles / 9h), 10:15 PM (5 cycles / 7.5h), or 11:45 PM (4 cycles / 6h). Pick the latest one that still gives you 5–6 cycles. Set the wake time in the tool to get your exact bedtimes.
Why add 15 minutes to fall asleep?›
Very few people fall asleep the instant their head hits the pillow — the average is 10–20 minutes (called sleep latency). Counting cycles from the moment you actually sleep, not the moment you lie down, makes the wake-up times more accurate. Adjust the slider to match yourself.
Does this account for naps or irregular schedules?›
No — it plans one continuous night of sleep. If you nap or do shift work, treat each sleep block separately. The 90-minute cycle principle still applies: try to wake at the end of a cycle for each block.