Sleep Calculator
Calculate the ideal bedtime or wake‑up time based on 90‑minute sleep cycles — wake up feeling refreshed.
Why 90 minutes? A full sleep cycle lasts about 90 minutes. Waking up mid‑cycle leaves you groggy. The calculator adds 15 minutes for the average time it takes to fall asleep.
How to Use Sleep Calculator
- Choose Mode A — enter the time you want to wake up, and the tool will show 6 ideal bedtimes (with 15 minutes to fall asleep).
- Alternatively, use Mode B — click If I sleep now to see wake‑up times based on the current time plus 15 minutes.
- Each suggested time shows the number of 90‑minute sleep cycles and total hours of rest.
- The best options (5 or 6 cycles) are highlighted, helping you choose a time that avoids waking mid‑deep‑sleep.
Benefits of Sleep Calculator
- Based on the scientifically accepted 90‑minute sleep cycle — waking between cycles reduces morning grogginess.
- Two modes cover both “what time should I go to bed?” and “when will I wake up if I sleep now?” scenarios.
- Highlights the two best cycle counts (5 or 6 cycles), removing the guesswork from choosing a bedtime.
- No data collection: the tool calculates times instantly in your browser without any account or server communication.
Frequently Asked Questions
Why are sleep cycles 90 minutes?
A full sleep cycle — from light sleep to deep sleep and REM — averages 90 minutes for adults. Waking up at the end of a cycle feels naturally refreshing, while waking mid‑cycle causes sleep inertia.
What is the 15‑minute falling‑asleep adjustment?
The average person takes 10–20 minutes to fall asleep after lying down. The tool adds 15 minutes to ensure the sleep‑cycle calculation starts from when you’re actually asleep, not just in bed.
Can I use the sleep calculator for shift work?
Yes, just enter the time you need to wake up before your shift, regardless of whether it’s morning or night. The calculator only cares about the wake‑up time, not the time of day.
Is Sleep Calculator free and private?
Yes — it is 100% free, needs no sign-up, and everything runs in your browser; your data never leaves your device.