Sleep Calculator
It works out what time to go to bed for a given wake-up time, or when to wake if you go to bed now, landing the alarm between 90-minute sleep cycles rather than in the middle of one.
Best bedtime
9:45 PM
Cycle length
90 min
Time to fall asleep
15 min
Recommended
5–6 cycles
That is
7.5–9 hours
Most adults need seven to nine hours, which is five or six full cycles. Consistency matters more than perfect timing — the same bedtime every night beats one carefully calculated exception.
What it calculates
It works out what time to go to bed for a given wake-up time, or when to wake if you go to bed now, landing the alarm between 90-minute sleep cycles rather than in the middle of one.
Why it matters
Waking mid-cycle, especially out of deep sleep, produces the groggy feeling known as sleep inertia. Six full cycles often leaves you sharper than seven broken hours.
Who it's for
Anyone with an early alarm, shift workers, and people who wake up tired despite spending long enough in bed.
Formula
- Cycle
- One full sleep cycle, about 90 minutes
- Latency
- Time to fall asleep, about 15 minutes
- Cycles
- Whole cycles targeted, usually 5 or 6
Worked example
Waking at 07:00
- 1Six cycles = 6 × 90 = 540 minutes, or 9 hours
- 2Add 15 minutes to fall asleep
- 3Count back 9h 15m from 07:00
Go to bed at 21:45 for six full cycles
How the sleep calculator works
Sleep runs in cycles of roughly 90 minutes, moving from light sleep into deep sleep and then REM. Waking at the end of a cycle, when sleep is lightest, feels far easier than being pulled out of deep sleep partway through. The calculator counts whole cycles back from your alarm and adds about fifteen minutes for the time it takes to actually fall asleep. Cycle length varies between people and across the night, so treat the times as targets rather than precision instruments.
Sleep runs in cycles of roughly 90 minutes, moving from light sleep into deep sleep and then REM. Waking at the end of a cycle, when sleep is lightest, feels far easier than being pulled out of deep sleep partway through.
Cycle length genuinely varies — roughly 70 to 120 minutes between people and across the night — so treat these as useful targets rather than precision instruments.
Common mistakes
- Treating 90 minutes as exact — real cycles run from about 70 to 120 minutes and vary through the night.
- Optimizing cycles while ignoring total sleep; five cycles is 7.5 hours, which is a reasonable adult target.
- Assuming a perfectly timed alarm fixes chronic sleep deprivation.
Tips and best practice
- Consistency matters more than timing — the same bedtime nightly beats a perfectly calculated one-off.
- If you routinely take much longer than fifteen minutes to fall asleep, adjust the latency to match.
- Most adults need seven to nine hours, which is five to six full cycles.
Frequently asked questions
What time should I go to bed to wake up at 7am?
For six full cycles, about 21:45 — nine hours of sleep plus roughly fifteen minutes to fall asleep. Five cycles would mean 23:15.
How long is a sleep cycle?
About 90 minutes on average, though it genuinely varies from roughly 70 to 120 minutes between people and lengthens as the night goes on.
Why do I feel worse after more sleep?
Usually sleep inertia from waking mid-cycle, especially out of deep sleep. Waking at the end of a cycle generally feels easier even with slightly less total sleep.
How many hours of sleep do I need?
Most adults need seven to nine hours, which is five to six full cycles. Teenagers typically need more, older adults sometimes slightly less.
Is the 90-minute rule scientifically exact?
No. It is a useful average, not a precise measure. Cycle length varies by person and by night, so treat these times as a helpful target rather than a rule.
Related calculators
Methodology & trust
- Formula source
- Standard 90-minute sleep cycle model with 15-minute sleep latency.
- Last updated
- 2026-07-28
- Privacy
- Every calculation runs in your browser. No inputs are sent to a server or stored.
- Accessibility
- Keyboard navigable, labeled inputs and WCAG AA color contrast.