About Sleep Calculator

Sleep Calculator is a free tool that helps you find a good time to go to bed or wake up, based on how sleep actually works: in repeating cycles rather than one continuous block.

Why we built this

Most people set an alarm around a round number — 7 hours, 8 hours — without accounting for where that lands within a sleep cycle. Waking up in the middle of deep sleep is a common reason people feel groggy even after what looks like "enough" sleep on paper. We built this calculator to make it simple to line up your wake-up time or bedtime with the natural boundaries between cycles instead.

How the calculation works

The tool assumes an average sleep cycle length of about 90 minutes and an average of 15 minutes to fall asleep, then works backward or forward from the time you enter to suggest options that land between cycles. You can find more detail on the calculator page itself, including a breakdown by age group and answers to common questions.

Who this is for

Anyone trying to time a bedtime or wake-up more thoughtfully — whether that's setting an alarm for tomorrow, planning around an early flight, or just trying to feel less groggy in the morning. It's a general planning tool, not a medical device, and it won't account for individual differences in sleep patterns, medical conditions, or sleep disorders.

A note on accuracy

Sleep cycle length varies from person to person and even night to night. The numbers here are population averages meant to give you a strong starting estimate, not a guarantee. If you're dealing with ongoing sleep difficulties, we'd encourage speaking with a healthcare professional rather than relying on this tool alone.