Sleep Calculator
Find the best time to sleep or wake up based on 90-minute sleep cycles. Wake up refreshed, not groggy.
If you fall asleep right now, these are the best times to wake up (between sleep cycles, so you feel refreshed).
How to Use the Sleep Calculator
- Choose I want to wake up at... and set your alarm time to get recommended bedtimes.
- Or choose I am going to sleep now to see the best times to set your alarm.
- Pick a time marked Recommended — 5 or 6 full 90-minute sleep cycles plus 15 minutes to fall asleep.
Formula & Method
Sleep happens in roughly 90-minute cycles moving through light sleep, deep sleep, and REM. Waking mid-cycle (especially during deep sleep) causes grogginess, while waking between cycles feels natural.
Bedtime = Wake time - (Cycles x 90 min + 15 min to fall asleep)
Adults need 4-6 cycles (6-9 hours) per night. The calculator subtracts 3, 4, 5, and 6 cycles from your wake time.
Examples
| Wake-up Time | 6 Cycles (9h) | 5 Cycles (7.5h) | 4 Cycles (6h) |
|---|---|---|---|
| 6:00 AM | 8:45 PM | 10:15 PM | 11:45 PM |
| 7:00 AM | 9:45 PM | 11:15 PM | 12:45 AM |
| 8:00 AM | 10:45 PM | 12:15 AM | 1:45 AM |
Frequently Asked Questions
What is a sleep cycle?
A sleep cycle is about 90 minutes long and moves through light sleep, deep sleep, and REM (dream) sleep. You go through 4-6 cycles per night.
Why do I feel tired after 8 hours of sleep?
You probably woke up mid-cycle during deep sleep. 8 hours is 5.3 cycles — try 7.5 hours (5 full cycles) or 9 hours (6 cycles) instead and you will likely wake up fresher.
Is 6 hours of sleep enough?
6 hours equals 4 full cycles — workable short-term for some people, but most adults need 7-9 hours. Chronic 6-hour sleep is linked to reduced focus and health issues.
How long does it take to fall asleep?
The average person takes 10-20 minutes. This calculator uses 15 minutes. If you regularly take much longer, reduce screen time and caffeine before bed.
What time should I go to bed to wake up at 7 AM?
For 5 full cycles, fall asleep by 11:15 PM (be in bed by 11:00 PM). For 6 cycles, fall asleep by 9:45 PM.
People Also Ask
Is it better to sleep 7.5 or 8 hours?
7.5 hours is exactly 5 sleep cycles, so you wake between cycles feeling refreshed. 8 hours lands mid-cycle for many people, causing grogginess despite more sleep.
How many sleep cycles do I need per night?
Most adults need 5-6 complete cycles (7.5-9 hours). Teens need more, and older adults may manage with slightly less.
Does waking up mid-cycle really matter?
Yes. Waking during deep sleep causes sleep inertia — heavy grogginess that can last 30-60 minutes. Waking between cycles, even with less total sleep, usually feels better.