Sleep Calculator

Find the best bedtime or wake-up time based on 90-minute sleep cycles. Waking at the end of a cycle leaves you feeling refreshed — waking mid-cycle causes grogginess.

Part of our Health Calculators collection.

Sleep Calculator

Free online calculator

I want to calculate...

I plan to fall asleep at

I need to wake up at

Results update automatically as you type

How to Use This Calculator

1
Select whether you want to find a wake-up time or bedtime.
2
Enter either when you plan to sleep or when you need to wake up.
3
The calculator shows multiple optimal times based on complete 90-minute cycles.
4
Choose the option that fits your schedule — 5 or 6 cycles is ideal for most adults.

How Sleep Cycles Work

One sleep cycle = ~90 minutes
Recommended: 5–6 complete cycles (7.5–9 hours)

If sleeping at 11:00 PM (add 15 min to fall asleep):
4 cycles → wake at 5:00 AM (6 hrs)
5 cycles → wake at 6:30 AM (7.5 hrs) ✓ Ideal
6 cycles → wake at 8:00 AM (9 hrs) ✓ Ideal

Example Calculation

Example: Falling asleep at 11:00 PM

Inputs

calcMode: wakeupsleepTime: 11:00 PMwakeTime: 7:00 AM

Result

Best wake times: 6:15 AM (5 cycles) or 7:45 AM (6 cycles)

Accounting for ~15 minutes to fall asleep, 5 complete 90-min cycles end at 6:15 AM; 6 cycles end at 7:45 AM. Both leave you refreshed.

Frequently Asked Questions

Why do I feel worse after more sleep?

Sleep inertia — waking mid-cycle is the culprit. If you wake after 8.5 hours but mid-cycle, you feel groggy. If you wake after 7.5 hours at the end of a cycle, you feel refreshed. The calculator shows you the cycle endpoints.

How many hours of sleep do adults need?

Adults need 7–9 hours per night. That's 5–6 complete 90-minute cycles. Consistently sleeping under 6 hours increases risk of obesity, diabetes, cardiovascular disease, and cognitive decline.

What if I can't fall asleep in 15 minutes?

If you often lie awake for 30–45+ minutes, adjust the bedtime earlier. Chronic difficulty falling asleep may indicate sleep hygiene issues: avoid screens 1 hour before bed, keep the room cool and dark, and wake at the same time every day.