Why is waking up at the end of a 90-minute cycle important?

Waking up in the middle of deep Stage 3/4 sleep causes sleep inertia (dizziness and brain fog). Waking up at the end of a 90-minute REM cycle ensures you feel fresh and energetic.

Why is 7.5 hours often better than 8 hours of sleep?

7.5 hours completes exactly 5 full 90-minute sleep cycles, whereas 8 hours interrupts you midway through your 6th cycle during deep sleep.

Is any personal data stored on servers?

No, never. All sleep calculations execute 100% locally in your browser memory.

Everyday Tools

Sleep Cycle & REM Bedtime Calculator

Interactive in-browser Sleep Cycle and REM Bedtime Calculator based on natural 90-minute ultradian sleep cycles and 14-minute sleep latency. Discover when to fall asleep or wake up to eliminate morning grogginess (sleep inertia). Features Sleep Now, Wake Time, and Power Nap modes with 1-click PNG schedule cards.

100% In-Browser Privacy

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How to Calculate Your Ideal Sleep Cycle in 3 Steps

Choose Calculation Mode

Select whether you want to wake up at a specific hour, sleep right now, or take a power nap.

Set Target Time & Latency

Choose your target alarm time and adjust your average time to fall asleep (default 14 min).

Set Your Alarm for Cycle Ends

Pick the optimal 5-cycle (7.5h) bedtime and download your personalized sleep card.

Frequently Asked Questions

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