Zzzz🌙Sleep Calculator✦.･*

When we sleep, our brain goes through several sleep cycles. Each cycle lasts about 90 minutes, during which we move from light sleep to deep sleep, and then to rapid eye movement (REM) sleep. If we’re woken in the middle of a cycle, we usually feel tired and groggy. But if we wake up between cycles, we feel refreshed. This calculator helps you find the best time to sleep or wake up by adding the average **15 minutes** it takes to drift off, then counting back or forward in 90-minute cycles.

CycleSleep adds 15 minutes for you to drift off, then counts backwards in 90-minute cycles. If you’re asleep by 11 pm, aim to wake up at either 5:15 am (5 cycles) or 6:45 am (6 cycles). You’ll rise between cycles and feel far less groggy.

Sleep inertia is that heavy, foggy feeling when you’re woken mid-cycle. To avoid it: use a cycle-aware alarm (like CycleSleep!) rather than a fixed clock time. Let sunlight in immediately — light suppresses melatonin quickly. Drink a glass of water; mild dehydration worsens grogginess. If needed, try a caffeine nap: coffee plus a 20-minute nap, no longer.

Each full cycle averages 90 minutes. Six hours equals 360 minutes, so that’s exactly four cycles. You’ll wake up refreshed if you align the end of a cycle with your alarm.

Four cycles (~6 h) can work if you’re healthy and catch additional rest later, but most adults feel their best at five to six cycles (7.5–9 h). Pay attention to your daytime energy, mood and focus — those are better guides than clock hours alone.