Sleep Cycle

About Sleep Cycle

Sleep Cycle is a free tool that answers one small but maddening question: what time should I actually go to bed — or wake up — so I don't feel like a zombie?

Everything runs in your browser. You tell it when you need to wake up (or that you're going to bed now) and roughly how long you take to fall asleep, and it suggests times that line up with whole sleep cycles. Nothing you type is sent anywhere or stored on a server.

How it works

Sleep moves through cycles of light sleep, deep sleep and REM that last about 90 minutes each. Most people complete four to six cycles a night. Waking at the end of a cycle — in light sleep — tends to feel far easier than being pulled out of deep sleep mid-cycle, which is the heavy, foggy feeling known as sleep inertia.

So instead of counting fixed hours, the calculator counts whole cycles. It also adds the time it takes you to fall asleep (about 15 minutes for most people, adjustable with a slider) before the first cycle begins, so the suggested times reflect real sleep rather than just lights-out.

Where the numbers come from — and their limits

The 90-minute average is well established, but real cycles vary from roughly 80 to 120 minutes and shift across the night and with age. This tool is a planning estimate, not a sensor reading — we don't measure your actual sleep stages. Treat the times as a helpful starting point, not a prescription.

We're not doctors, and this site doesn't provide medical advice. If you have ongoing trouble sleeping, persistent fatigue, or a suspected sleep disorder, please talk to a qualified healthcare professional.

Who makes it

Sleep Cycle is an independent project built and maintained by zhiyiWorld. It exists because the existing sleep calculators were either cluttered, slow, or unpleasant to use — and a simple question deserved a calmer answer. If you have feedback or spot a calculation you think is wrong, the contact page is the fastest way to reach us.

Open the calculator →