RestCycles.com is a free sleep calculator built around one simple idea: waking up refreshed has nothing to do with how long you sleep — it's about when you wake up.
Most people set an alarm for 7 or 8 hours and hope for the best. But sleep happens in 90-minute cycles, and if your alarm goes off in the middle of one of those cycles, you'll feel groggy no matter how many hours you logged. Wake up between cycles — at the lightest point in your sleep — and you'll feel alert from the moment you open your eyes.
RestCycles does one thing really well: it takes the science of sleep cycles and turns it into a simple, practical answer. You tell us when you need to wake up. We tell you the best time to go to sleep. No sign-up, no app to download, no nonsense.
The tools on this site are based on the well-established 90-minute sleep cycle model, which has been studied extensively in sleep science research. Each cycle moves through light sleep, deep slow-wave sleep, and REM (rapid eye movement) sleep. Waking during deep sleep — which happens in the middle of a cycle — triggers sleep inertia, the groggy foggy feeling that can last for hours. Waking during light sleep, at the end of a cycle, is a much gentler transition back to consciousness.
The 15-minute "fall asleep" buffer built into our calculations is based on average sleep onset latency for healthy adults. If you tend to fall asleep faster or slower, you can mentally adjust the suggested times by a few minutes.
No — RestCycles is a general wellness tool, not a medical resource. If you're struggling with chronic insomnia, sleep apnea, or other sleep disorders, please consult a physician or sleep specialist. This calculator is designed to help healthy adults optimize their sleep timing, not to diagnose or treat any condition.
RestCycles is an independent project built by a small team passionate about practical wellness tools. We believe useful things should be free, fast, and free of unnecessary clutter. Every tool on this site runs entirely in your browser — nothing you enter is ever stored or sent anywhere.
Have a suggestion or found a bug? We'd love to hear from you at hello@restcycles.com or visit our contact page.