Sleep Calculator

What is a sleep calculator?

A sleep calculator works out the best times to fall asleep or wake up so that your alarm lines up with the natural rhythm of your sleep cycles. Sleep is not uniform: it moves through repeating cycles of about 90 minutes, each running from light sleep into deep sleep and then REM. Waking at the end of a cycle, when sleep is lightest, feels much easier than being pulled out of deep sleep — which is why two people who both slept seven hours can feel completely different in the morning. Enter the time you want to wake up (or the time you plan to go to bed) and the calculator lists wake-up or bedtime options built from 4, 5 and 6 complete cycles, with the most restorative 7.5- and 9-hour options highlighted.

How to use it

1. Choose a mode: "I want to wake up at" if you have a fixed alarm time, or "I'm going to bed at" if you want to know when to set the alarm. 2. Set the time, or tap "Now" to use the current time. The calculator instantly lists the suggested times for 4, 5 and 6 sleep cycles, each labelled with the total sleep it gives. Aim for one of the highlighted recommended options (5 or 6 cycles) when you can, and use the 4-cycle option only for a short night.

How it works: sleep cycles and onset time

Sleep runs through cycles of roughly 90 minutes. Within each cycle the brain passes through light NREM sleep, deeper NREM sleep used for physical recovery, and REM sleep, where most dreaming and memory consolidation happen. Across a full night the proportion of deep sleep falls and REM grows, so later cycles feel lighter. The calculator also adds a sleep-onset latency of about 14 minutes — the average time it takes to actually fall asleep after getting into bed. Wake time = bedtime + 14 min + (number of cycles × 90 min) Bedtime = wake time − (number of cycles × 90 min) − 14 min For example, going to bed at 22:00 gives wake-up options at 05:44 (5 cycles, 7.5 hours) and 07:14 (6 cycles, 9 hours). Times wrap correctly past midnight.

Reading your results

Each card shows a suggested time, the number of completed 90-minute cycles, and the total time asleep. The highlighted cards (5 and 6 cycles, 7.5 and 9 hours) match the amount of sleep recommended for most adults. The 4-cycle option (6 hours) is a fallback for nights when you can't get a full rest. Because real cycles vary from person to person and night to night, treat the times as a guide: a few minutes either way still lets you wake near the light end of a cycle, which is the whole point.

Frequently asked questions