Sleep Calculator
Want to wake up less groggy? Pick when you need to wake up and you will see the best bedtimes in 90 minute cycles. You will fall asleep faster and feel fresher.
TL;DR
- A sleep cycle lasts about 90 minutes and repeats 4 to 6 times per night
- Sleep Cycle Calculation Formulas: Bedtime = Target Wake Time - (Cycles × 90 min) - Latency (14 min)
- Example 1: Waking up refreshed at 07:00 AM
How it works
A sleep cycle lasts about 90 minutes and repeats 4 to 6 times per night. Early cycles have deep sleep for recovery, later cycles have more REM for memory.
The math
Count back in 90 minute blocks from your wake time and subtract 14 minutes to fall asleep.
Counting backwards or forwards in 90-minute intervals ensures the alarm rings during N1/N2 light sleep when brainwaves are fast, allowing you to wake up alert and energized.
Examples
Example 1: Waking up refreshed at 07:00 AM
- Target wake-up time: 07:00 AM
- Average fall-asleep latency: 14 minutes
- 5 Cycles (7.5 hours of sleep): 07:00 AM - 7h 30m - 14m = 11:16 PM bedtime (Recommended)
- 6 Cycles (9.0 hours of sleep): 07:00 AM - 9h 00m - 14m = 09:46 PM bedtime (Optimal Recovery)
Example 2: Going to bed at 11:30 PM
- Bedtime: 11:30 PM
- Estimated time to fall asleep: 11:44 PM (+14 minutes)
- 4 Cycles (6.0 hours): Wake up at 05:44 AM
- 5 Cycles (7.5 hours): Wake up at 07:14 AM (Optimal)
- 6 Cycles (9.0 hours): Wake up at 08:44 AM
When to use it
✦ Morning Vitality & Productivity
Eliminate morning brain fog by synchronizing alarms with the light sleep phase at the end of a 90-minute cycle.
✦ Strategic Power Napping
20-minute power naps for quick alertness or 90-minute full cycle naps for memory and creativity without post-nap grogginess.
✦ Managing Sleep Debt Deficits
Calculate weekly accumulated sleep debt and recover safely without disrupting circadian rhythms.
⚠ Common mistakes
- Hitting the Snooze Button: Hitting snooze causes your brain to start a new 90-minute cycle that is repeatedly interrupted, causing severe morning grogginess.
- Overusing Binge Sleep on Weekends: Sleeping 3+ extra hours on weekends shifts your central circadian pacemaker, creating 'social jetlag' on Monday morning.
FAQ (6)
What happens if I wake up in N3 deep sleep?
Waking up during N3 deep sleep triggers sleep inertia, making you feel exhausted and sluggish even if you slept for hours.
Is 6 hours of sleep enough for an adult?
While 6 hours (4 cycles) is manageable short-term, long-term health requires 7.5 to 9 hours (5 to 6 cycles) for full REM and physical repair.
What happens to my weight, height or other health data?
It never leaves your phone. BMI, BMR, body-fat — all computed on-device, no upload, no account. That's intentional for health data: you can check a BMI of 22.8 or a TDEE of 2,100 kcal without worrying about a server log. Closing the tab clears the form from memory.
Does it work offline at the gym or on a plane?
Yep. Once the page has loaded, the calculators are fully offline-first. I've used the pace and BMI tools in airplane mode between flights — no spinner, no retry, just instant numbers. Same for sleep cycles if you're planning a shift.
Can my doctor rely on these numbers?
They're solid estimates using published formulas (WHO for BMI, Mifflin-St Jeor for BMR, CKD-EPI for GFR) but they're not a diagnosis. Think of them as a conversation starter: good for tracking trends, but your GP or nephrologist should interpret GFR, heart age or pregnancy dates with labs and context. We link the sources at the bottom for that reason.
How can I share my result with a trainer or doctor?
Use 'Share Link' in the results panel. It builds a URL with your inputs — for example BMI 70kg/175cm — so they see exactly what you entered. No data is stored on our side, the link just carries the numbers.