Feel groggy in the morning sometimes? That's called sleep inertia, it means you've woken up during a sleep cycle.

Sleep Cycle App was created to help you sleep better. It takes into account how long it takes the average human to fall asleep and then tells you when to wake up based on REM sleep cycles.

This calculator aims to help you avoid this by counting backwards in sleep cycles of 90 minutes to help you wake up feeling refreshed.
A sleep cycle lasts about 90 minutes and a good night's sleep consists of 5-6 sleep cycles. If you wake up in the middle of a sleep cycle, you will feel groggy even if you've completed several cycles prior to waking up.