Sleep Calculator
Find the best wake-up times using 90-minute sleep cycles from now or bedtime.
Written by Golam Rabbani, Founder & Lead Engineer — Last updated 2026-05-01
How to use this sleep calculator
- Choose a mode: "Wake up at?" to find wake times, or "Sleep at?" to find bedtimes.
- Enter your planned bedtime or required wake-up time, or press Now to use the current time.
- Press Calculate to see four recommended times based on complete sleep cycles.
- Read the result panel and use Copy to save the times, or Reset to start over.
About this sleep calculator
The sleep calculator tells you the best times to wake up or go to bed so that you complete a whole number of 90-minute sleep cycles rather than waking mid-cycle.
Human sleep is structured in repeating cycles of roughly 90 minutes each, moving through light sleep, deep sleep, and REM. Waking at the end of a cycle — rather than in the middle of deep or REM sleep — is associated with feeling more refreshed. The sleep calculator adds 14 minutes to your chosen bedtime to account for the average time it takes to fall asleep, then adds 3, 4, 5, or 6 full cycles (270, 360, 450, or 540 minutes) to produce four candidate wake times. The reverse mode works backwards from a required wake time to give four corresponding bedtimes. This is an estimate for general information and is not a substitute for advice from a qualified clinician.
For example, if you plan to go to bed at 10:30 PM, the tool adds 14 minutes (placing sleep onset at 10:44 PM) and then adds 6 cycles × 90 minutes = 540 minutes, returning a wake time of 6:44 AM for 9 hours 14 minutes of time in bed. The 5-cycle option gives 5:14 AM (7 hours 44 minutes), and so on. Anyone wanting to optimise their morning energy — students, shift workers, parents with early starts — can use these options to plan a consistent sleep schedule. This calculator is for general educational purposes only and is not medical advice — consult a licensed healthcare professional for personal guidance.
FAQ
- What does the sleep calculator do?
- It calculates the ideal times to wake up or go to bed based on 90-minute sleep cycles. Given a bedtime, it shows four wake-up options (3 to 6 cycles); given a wake time, it shows four matching bedtimes.
- What formula does the sleep calculator use?
- It adds 14 minutes to the bedtime (the average sleep-onset latency) and then adds multiples of 90 minutes — one full sleep cycle — for 3, 4, 5, and 6 cycles. The reverse mode subtracts the same total from the desired wake time.
- How accurate is the sleep cycle estimate?
- Sleep cycle length varies between individuals and across a single night, averaging 90 minutes but ranging roughly 80–110 minutes. The 14-minute fall-asleep figure is a population average. Treat the results as a useful guideline rather than a precise prescription.
- What happens if I enter no time and press Calculate?
- The tool shows an error message asking you to select a time. Press Now to fill the field with the current time automatically, then press Calculate.
- Does the sleep calculator store my data?
- No. The sleep calculator runs entirely in your browser. No times or personal data are sent to a server or saved between sessions.
- Is the sleep calculator free?
- Yes, it is completely free with no signup or account required.