Sleep Calculator
Find the best time to go to bed or wake up using 90-minute sleep cycles, so you wake between cycles feeling refreshed — in line with the NHS 7–9 hour adult guide.
Sleep cycle planner
90-minute cycles + 15 min to fall asleep.
A guide based on 90-minute cycles. Most adults need 7–9 hours — see NHS — sleep and tiredness.
How sleep cycles work
Sleep runs in cycles of roughly 90 minutes, moving from light sleep into deep sleep and REM, then back to light. Waking at the end of a cycle — in light sleep — feels far easier than being jolted awake mid-deep-sleep. This calculator counts back (or forward) in 90-minute blocks, adding about 15 minutes to fall asleep, so you can aim to wake between cycles.
To wake at 7:00 am, good bedtimes are about 9:45 pm, 11:15 pm or 12:45 am — giving 6 or 7.5 hours of sleep across complete cycles (plus 15 minutes to drift off). Most adults do best on 5 to 6 cycles (7.5–9 hours).
How much sleep adults need
| Age group | Recommended sleep |
|---|---|
| Adults (18–64) | 7–9 hours |
| Older adults (65+) | 7–8 hours |
| Teenagers | 8–10 hours |
| One sleep cycle | ≈ 90 minutes |
Source: NHS — sleep and tiredness.
Better sleep, not just more
Cycle timing is a helpful nudge, but consistency matters more: going to bed and waking at similar times every day sets your body clock. Keep the bedroom dark and cool, avoid screens and caffeine before bed, and get daylight in the morning. The 90-minute figure is an average — yours may run a little shorter or longer — so treat the suggested times as a starting point.
Frequently asked questions
How does a sleep calculator work?
It counts sleep in 90-minute cycles. From your wake-up time it subtracts whole cycles (plus time to fall asleep) to suggest bedtimes, so your alarm goes off at the end of a cycle in light sleep rather than mid-deep-sleep.
How many hours of sleep do I need?
The NHS recommends most adults aged 18–64 get 7 to 9 hours a night. In sleep cycles that's about 5 to 6 complete 90-minute cycles.
Why do I feel worse after 8 hours sometimes?
If your alarm interrupts a deep-sleep stage, you can wake groggy even after plenty of sleep. Timing your wake-up to the end of a 90-minute cycle usually feels more refreshing than a fixed number of hours.
Is the 90-minute cycle exact?
No — it's an average. Real cycles range from about 70 to 110 minutes and change through the night. Use the suggested times as a guide and adjust based on how you feel.
What's the best way to improve my sleep?
Keep a regular sleep schedule, make your bedroom dark, quiet and cool, avoid screens and caffeine before bed, and get natural light in the morning. Consistency tends to help more than chasing a perfect bedtime.