Sleep Calculator
Find the best time to go to bed or wake up based on 90-minute sleep cycles. Get optimized sleep schedules so you wake up feeling refreshed and alert.
How to Use This Sleep Calculator
- Choose your mode: "I want to wake up at" or "I want to fall asleep at".
- Select the hour, minutes, and AM/PM.
- Click Calculate Sleep Times to see recommended times.
- Pick a time that gives you a full number of 90-minute sleep cycles (4–6 cycles is ideal).
Formula
Recommended Time = Target Time − (Cycles × 90 min) − 14 min (fall asleep)
Each complete sleep cycle lasts approximately 90 minutes, progressing through light sleep, deep sleep, and REM sleep. It takes the average person about 14 minutes to fall asleep. Waking up mid-cycle causes sleep inertia (grogginess).
Examples
If you want to wake up at 7:00 AM:
6 cycles: 7:00 AM − 9h = 10:00 PM
5 cycles: 7:00 AM − 7.5h = 11:30 PM
4 cycles: 7:00 AM − 6h = 1:00 AM
If you want to fall asleep at 11:00 PM:
6 cycles: 11:00 PM − 9h = 2:00 PM (next day wake)
5 cycles: 11:00 PM − 7.5h = 3:30 PM
4 cycles: 11:00 PM − 6h = 5:00 PM
Frequently Asked Questions
What are sleep cycles?
Sleep cycles are the natural progression through different stages of sleep: light sleep (N1, N2), deep sleep (N3), and REM (Rapid Eye Movement) sleep. Each complete cycle lasts approximately 90 minutes. Adults typically need 4–6 cycles per night (6–9 hours).
How much sleep do I need?
Most adults need 7–9 hours of sleep per night, which equals about 5–6 complete sleep cycles. Teenagers need 8–10 hours, and older adults (65+) may function well with 7–8 hours. Quality matters as much as quantity.
What is the best time to go to bed?
The best bedtime is one that allows you to complete 5–6 sleep cycles before your desired wake time. Most adults naturally feel sleepy between 10:00 PM and midnight. Maintaining a consistent sleep schedule, even on weekends, helps regulate your circadian rhythm.
Why do I feel groggy when my alarm goes off?
Grogginess (sleep inertia) occurs when you wake up during deep sleep rather than at the end of a sleep cycle. Using this calculator to time your sleep for the end of a cycle helps you wake up during lighter sleep stages, reducing morning grogginess.