Sleep Calculator
Enter a wake-up time, or calculate from right now, and get several bedtime or wake-up options built from natural 90-minute sleep cycles.
This calculator is built around a simple idea: sleep happens in repeating cycles of roughly 90 minutes, and waking up between cycles feels very different from waking up in the middle of one. Enter the time you need to be awake and it counts backward in full 90-minute blocks, plus the roughly 15 minutes most people take to fall asleep, to suggest several practical bedtimes. Switch to "I'm going to sleep now" and it counts forward from the current time instead, suggesting wake-up times that line up with the end of a full cycle. Everything runs locally in your browser the moment you type or tap.
Worked examples
Catching a 5 a.m. airport shuttle
A traveler sets an alarm for 5:00 AM to make an early flight and wants a bedtime that avoids waking up mid-cycle after a short night.
- Wake up
- 5:00 AM
- Cycles
- 4
- Buffer
- 15 min
Go to bed by 10:45 PM for 6h of full-cycle sleep
Turning in after a midnight shift ends
A nurse finishes a shift at 12:15 AM and wants to know the best times to be woken up if they fall asleep shortly after getting home.
- Falling asleep
- 12:15 AM
- Cycles
- 5
- Buffer
- 15 min
Wake up around 8:00 AM after 7.5h of full-cycle sleep
How sleep cycles work
Overnight sleep isn't uniform — it moves through repeating cycles that last about 90 minutes each, dipping from light sleep into deep, slow-wave sleep and back up into REM (rapid eye movement) sleep, where most dreaming happens. Early cycles in the night lean heavily toward deep sleep, the stage most associated with physical recovery, while later cycles shift toward more REM. Getting pulled out of a cycle partway through, especially during deep sleep, tends to produce "sleep inertia" — grogginess that can linger for 20-30 minutes — while waking up between cycles, when sleep is naturally lighter, tends to feel far more alert.
This calculator works backward or forward in whole 90-minute blocks and adds roughly 15 minutes for the average time it takes to fall asleep once in bed, then shows several options instead of a single number, since the right cycle count still depends on how much total sleep is realistic for the night.
Napping without wrecking your night
Power nap — about 20 minutes
Caps out in light sleep, before the body drops into deep sleep, so waking up is fast and largely free of grogginess. Good for an early- to mid-afternoon energy dip.
≈ 20 min, minimal sleep inertia on waking
Full-cycle nap — about 90 minutes
Completes one entire sleep cycle, deep and REM sleep included, so waking happens near the top of a cycle rather than in the middle of one. A bigger recovery boost, at the cost of a full hour and a half.
≈ 90 min, one complete sleep cycle
Naps in between — roughly 45 to 60 minutes — tend to end in the middle of deep sleep, which is why they often feel worse than either a short or a full-cycle nap.
How much sleep do you actually need?
| Age group | Recommended sleep per 24h |
|---|---|
| Newborn (0–3 months) | 14–17 hours |
| Infant (4–11 months) | 12–15 hours |
| Toddler (1–2 years) | 11–14 hours |
| Preschool (3–5 years) | 10–13 hours |
| School age (6–13 years) | 9–11 hours |
| Teen (14–17 years) | 8–10 hours |
| Young adult (18–25 years) | 7–9 hours |
| Adult (26–64 years) | 7–9 hours |
| Older adult (65+ years) | 7–8 hours |
Ranges reflect general guidelines published by sleep-health organizations such as the U.S. National Sleep Foundation and cover total sleep across 24 hours, naps included for infants and young children.
Frequently asked questions
Why does this calculator use 90-minute cycles instead of just a total number of hours?
Sleep isn't a flat, uniform state — it moves through repeating cycles of light, deep, and REM sleep, and one full cycle takes about 90 minutes for most adults. Waking up in the middle of a cycle, especially during deep sleep, tends to leave you groggy even if you technically got 'enough' hours. Counting in whole cycles instead of raw hours is meant to land your alarm near the light-sleep boundary between cycles, where waking up naturally feels easier.
Is the 15-minute fall-asleep estimate accurate for me specifically?
It's a population average, not a personal measurement — most healthy adults fall asleep somewhere between 10 and 20 minutes after getting into bed, so 15 minutes is a reasonable midpoint. If you know you typically take longer (or fall asleep almost instantly), mentally shift the suggested bedtime earlier or later by the difference. Over a few nights of testing the results against how you actually feel, you can dial in a personal offset.
What actually happens if I wake up in the middle of a cycle instead of between two?
Nothing dangerous — but you may experience "sleep inertia": a period of grogginess, slowed reaction time, and disorientation that can last anywhere from a few minutes to half an hour. It happens because you were pulled out of a deeper stage of sleep before your brain finished that stage naturally. This calculator tries to minimize that by targeting the boundaries between cycles rather than picking an arbitrary hour count.
Should I always aim for the "Recommended" 5 or 6 cycle options?
Five to six cycles (about 7.5 to 9 hours) covers the range most sleep guidelines recommend for adults, which is why those options are flagged. But total sleep need varies by person, age, and how much sleep debt you're carrying, so the shorter options aren't wrong — they're just trade-offs. Use them for nights where a full cycle count genuinely isn't possible, not as a routine target.
Do naps count as sleep cycles the same way nighttime sleep does?
The same 90-minute cycle logic applies, but naps are usually intentionally cut short before a full cycle completes. A roughly 20-minute nap keeps you in light sleep and lets you wake up quickly and clear-headed, while a 90-minute nap lets you complete one whole cycle, deep sleep and REM included, which trades a longer nap for a bigger cognitive boost. Napping for 45-60 minutes tends to land you mid-cycle, in deep sleep, which is why medium-length naps often feel worse than either short or long ones.
Why do the two tabs sometimes suggest overlapping but not identical times?
The two tabs solve different problems: "I need to wake up at" works backward from a fixed deadline, while "I'm going to sleep now" works forward from the current moment. If the current time happens to already line up with one of the backward-calculated bedtimes, the results can look similar or even match — but they aren't tied together, so treat each tab as answering its own question rather than expecting them to agree.