Sleep Calculator

Find the ideal bedtime or wake-up time based on your sleep cycles.

How this calculator works

Sleep happens in cycles of about 90 minutes, each moving through several stages (light sleep, deep sleep, REM sleep). Waking up mid-cycle, during deep sleep, often causes grogginess and confusion (sleep inertia), even after a full night. Aiming for the end of a cycle rather than a round number of hours increases the chances of waking up refreshed.

The calculation

Target time ± (number of cycles × 90 minutes + 15 minutes to fall asleep). The 5- and 6-cycle options (7h30 and 9h) are highlighted as the most recommended for an adult.

Frequently asked questions

What is a sleep cycle?

A sleep cycle lasts about 90 minutes and includes several stages: light sleep, deep sleep, and REM sleep (dreaming). A full night typically consists of 4 to 6 consecutive cycles.

Why is it better to wake up at the end of a cycle?

Waking up during deep sleep causes sleep inertia: a feeling of grogginess and confusion that can last several tens of minutes. Waking up at the transition between two cycles is generally gentler.

Where does the 15-minute estimate to fall asleep come from?

It's a commonly used average for a healthy adult. Actual time to fall asleep varies by person, stress level, or caffeine intake, among other factors.

How many sleep cycles are recommended per night?

For most adults, 5 to 6 full cycles (about 7h30 to 9h) are recommended. Needs vary, however, depending on age, physical activity, and individual health.

Does this calculator replace medical advice?

No. It's a purely informational tool based on general averages. For persistent sleep issues, consult a healthcare professional.

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