What is a sleep cycle?

A sleep cycle is roughly 90 minutes long and includes light sleep, deep sleep and REM. Waking up in the middle of a cycle often feels worse than waking at the end, even after less total sleep.

How many cycles do I need?

Most adults need 5–6 cycles (7.5–9 hours) per night. Athletes and teenagers often need more. Consistently under 4 cycles (6 hours) is linked to health risks.

Why add 15 minutes to bedtime?

Most people take around 15 minutes to fall asleep after getting into bed. The calculator adds this so your cycles line up with actual sleep onset, not lights-out.

Do I need to wake up at the exact cycle end?

No, but aim for it. A variation of 10–15 minutes still feels fine. If you regularly wake groggy, try shifting your bedtime by 15–30 minutes to land better in the cycle.

What happens if I wake up in the middle of deep sleep?

Waking during deep sleep (usually the first half of the night) produces strong sleep inertia — grogginess, disorientation and slower reaction time for up to an hour. Waking at the end of a cycle in light sleep feels much better.

Can I use this to plan naps?

Yes. For a short nap, aim for one full cycle (about 90 minutes including the 15-minute fall-asleep buffer). For a power nap, 20 minutes is better — waking before deep sleep starts avoids the groggy feeling.

Sleep Cycle Calculator

Work out the best time to go to bed based on when you need to wake up. The calculator aligns your bedtime with full 90-minute sleep cycles, so you finish a cycle in light sleep instead of waking up groggy in the middle of deep sleep.