Sleep Cycle Calculator
Find the best time to sleep or wake up based on 90-minute sleep cycles. Wake feeling refreshed by aligning with your natural sleep rhythm.
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Frequently Asked Questions
How does the Sleep Cycle Calculator work?
The tool uses a fixed 90-minute sleep cycle duration and adds a 15-minute fall-asleep buffer. For "wake at" mode, it subtracts multiples of 90 minutes from your target wake time (plus the buffer) to suggest optimal bedtimes after 3, 4, 5, or 6 complete cycles. For "sleep at" mode, it adds the buffer plus multiples of 90 minutes to your bedtime to suggest optimal wake times. All calculations use JavaScript Date arithmetic.
How do sleep cycles work?
Each sleep cycle lasts approximately 90 minutes and includes light sleep, deep sleep, and REM phases. Waking up at the end of a complete cycle (rather than in the middle) helps you feel more alert and less groggy.
How many sleep cycles do I need?
Most adults need 5-6 complete cycles (7.5 to 9 hours). The calculator shows multiple options so you can choose a bedtime or wake time that fits your schedule while completing full cycles.
Does it account for time to fall asleep?
Yes. The calculator factors in an average of 15 minutes to fall asleep. The suggested bedtimes account for this so the actual sleep time aligns with complete cycles.