How to use it
- Pick a mode: a fixed wake-up time (the most common case), a fixed bedtime, or “Sleep now”.
- Enter the time. The best option (5 cycles, 7.5 hours) is shown large; all four options are listed with the amount of sleep each gives.
- Adjust the time to fall asleep if you know yours – 15 minutes is the default, 10–20 is typical.
- Copy the times or the link (the link stores your chosen time and settings; “sleep now” results are not stored because they depend on the clock).
Example
You need to be up at 06:30. Five cycles are 5 × 90 = 450 minutes; add 15 minutes to fall asleep and count back 465 minutes: lights out at 22:45. Six cycles would mean 21:15, four cycles 00:15. Going the other way, if you get into bed at 23:00 and take 15 minutes to drift off, cycle ends fall at 03:45, 05:15, 06:45 and 08:15 – so an alarm at 06:45 is kinder than one at 06:00, which would ring in the middle of your fifth cycle.
How the times are calculated, and the limits of the cycle model
bedtime = wake-up time − cycles × 90 min − time to fall asleep (cycles = 6, 5, 4, 3)
wake-up time = bedtime + time to fall asleep + cycles × 90 min
Times wrap around midnight. 7–9 h recommended for adults 18–64; 7–8 h for 65+The 90-minute figure comes from polysomnography: a typical adult night contains four to six NREM–REM cycles averaging 90–110 minutes, with more deep sleep in the first cycles and more REM in the later ones (Carskadon & Dement, in Principles and Practice of Sleep Medicine). The length varies between people and from night to night, so the calculator cannot know where your cycle boundaries really fall – it gives a sensible target, not a guarantee of waking refreshed. The 15-minute sleep latency is also an average; falling asleep in under 5 minutes often signals sleep deprivation, and regularly taking more than 30 minutes is worth discussing with a doctor. The sleep-duration recommendations are those of the National Sleep Foundation expert panel (Hirshkowitz et al. 2015). Consistency – the same bedtime and wake-up time every day, including weekends – matters more for feeling rested than hitting an exact cycle boundary. Compared with the previous version, this one adds the bedtime mode, an adjustable sleep latency, a 12-hour clock and the amount of sleep per option.
Sources: Hirshkowitz et al. 2015, Sleep Health – National Sleep Foundation sleep duration recommendations, Patel, Rishi & Araujo – Physiology, Sleep Stages (StatPearls), CDC – About sleep.
This tool is for planning a normal night and is not medical advice. Persistent trouble falling or staying asleep, loud snoring or daytime sleepiness should be discussed with a doctor.
Why waking mid-cycle feels bad
Deep (slow-wave) sleep dominates the first half of the night. An alarm that goes off during it produces sleep inertia: grogginess, slowed reactions and low mood that can last 30 minutes or more. Late in the night most of each cycle is lighter sleep and REM, from which waking is quick. That is why a slightly earlier alarm at a cycle boundary can feel better than 20 extra minutes that end in deep sleep.
Questions
- What time should I go to bed to wake up at 6?
- With 15 minutes to fall asleep: 20:45 for 6 cycles (9 h), 22:15 for 5 cycles (7.5 h), 23:45 for 4 cycles (6 h). Enter 06:00 in the calculator for a 12-hour display or a different latency.
- Is 6 hours of sleep enough?
- Four cycles (6 h) is below the 7–9 hours recommended for adults. It is manageable occasionally, but regular 6-hour nights accumulate sleep debt and impair attention and metabolism.
- Are sleep cycles really 90 minutes?
- On average, yes – measured cycles span about 70 to 120 minutes and change through the night. Treat the suggested times as a guide and keep the same schedule every day; your body adapts to regularity.
- How does "Sleep now" work?
- It takes the current time from your device, adds the time to fall asleep and lists wake-up times after 3, 4, 5 and 6 cycles. It refreshes every 15 seconds while the page is open.