Sleep Calculator 😴
Find the best time to sleep or wake up based on natural 90-minute sleep cycles. Wake feeling refreshed, not groggy.
✓ Based on 90-min cycles✓ Includes time to fall asleep
Last updated: April 2026
Best times to go to sleep
Times based on average 90-minute sleep cycles plus 15 minutes to fall asleep. Individual sleep patterns vary. Adults generally need 7–9 hours per night for optimal health (NHS guidance).
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Why do sleep cycles matter?
Sleep occurs in cycles of approximately 90 minutes, each with several stages including light sleep, deep sleep, and REM. Waking at the end of a complete cycle — when you're naturally in a lighter stage — means you'll feel more alert and refreshed. Waking mid-cycle, especially during deep sleep, causes that groggy, disoriented feeling known as sleep inertia.
How many sleep cycles do you need?
Most adults need 5–6 complete sleep cycles per night (7.5–9 hours). The NHS recommends 7–9 hours per night for adults. Consistently getting less than 7 hours is associated with increased risk of obesity, heart disease, diabetes, and mental health issues.
Frequently asked questions
Is 6 hours enough?
For most adults, 6 hours (4 cycles) isn't enough on a regular basis. The NHS recommends 7–9 hours per night for adults.
Why do I feel more tired after 8 hours than 7.5?
You've likely woken up mid-cycle. 7.5 hours = 5 complete cycles, but 8 hours puts you 30 minutes into the 6th cycle, often during deep sleep. This is why cycle-based timing can be more useful than targeting round numbers.
Do sleep cycles last exactly 90 minutes?
No — 90 minutes is an average. Individual cycles range from 80 to 110 minutes, and cycles early in the night tend to be shorter. If suggested times don't work for you, try adjusting by 10–15 minutes.