Free · Science-Based · Instant
Sleep Calculator
Find your perfect bedtime or wake-up time using 90-minute sleep cycles. Wake up refreshed instead of groggy, by aligning your alarm with the end of a cycle.
Advanced settings
Most adults average 90 minutes per cycle (range: 60–120 min)
Typical sleep latency is 10–20 minutes
Enter your time and age, then tap Calculate to see your ideal sleep windows.
- Based on peer-reviewed sleep research
- Instant, private results, nothing leaves your device
- Tailored to your age, 0 to 65+
The Science
How sleep cycles work
Your brain doesn't sleep in one continuous block, it cycles through stages roughly every 90 minutes. Waking at the right point in that cycle is the difference between feeling refreshed and feeling foggy.
Light sleep
The transition zone, lasting just a few minutes. You can be woken easily and may not even realise you were asleep.
Stable sleep
Body temperature drops, heart rate slows. You spend roughly half the night in this stage across all cycles.
Deep sleep
Slow-wave sleep, physical restoration, immune function, growth hormone release. Waking here causes the worst grogginess.
Dream sleep
Brain activity surges, eyes move rapidly, dreams happen. Critical for memory consolidation and emotional processing.
By Age
How much sleep do you actually need?
Sleep needs change dramatically across the lifespan. The National Sleep Foundation's expert panel, a multidisciplinary review of hundreds of sleep studies, published the most widely cited guidelines, summarized below. The calculator above uses these ranges to highlight your recommended option.
| Age group | Recommended sleep | Cycles (90 min) |
|---|---|---|
| Newborns (0–3 months) | 14–17 hours | 9–11 |
| Infants (4–11 months) | 12–15 hours | 8–10 |
| Toddlers (1–2 years) | 11–14 hours | 7–9 |
| Preschool (3–5 years) | 10–13 hours | 7–9 |
| School-age (6–13 years) | 9–11 hours | 6–7 |
| Teens (14–17 years) | 8–10 hours | 5–7 |
| Young adults (18–25) | 7–9 hours | 5–6 |
| Adults (26–64) | 7–9 hours | 5–6 |
| Older adults (65+) | 7–8 hours | 5 |
Source: National Sleep Foundation duration consensus, Sleep Health 1(4):233–243 (2015).
Getting Started
How to use the sleep calculator
The calculator works backward (or forward) from a fixed point in your day, counting in 90-minute sleep cycles so your alarm lands during light sleep instead of deep sleep. Here's how to get the most out of it:
- Pick what you know. If you have to be up at a set time, choose "I want to wake up at" and enter your alarm time. If you know roughly when you'll get into bed, choose "I want to go to bed at." Heading to bed right now? Use "Sleep now" and it fills in the current time.
- Set your age. Sleep needs change a lot across life, so the calculator adjusts the recommended number of cycles for your age band. The recommended option is the highlighted one.
- Read the options. Each result shows a time, the number of complete cycles it gives you, and the total sleep. The highlighted chip is the best fit for your age; the others let you trade more sleep for a later bedtime or earlier wake.
- Fine-tune if needed. Open Advanced settings to adjust your personal cycle length (the 90-minute average varies from about 70 to 120 minutes between people) and how long you take to fall asleep (the default is 15 minutes).
Everything runs instantly in your browser. Nothing is sent anywhere, and your last entry is remembered on your device so you don't have to re-enter it next time.
Why It Works
Why waking up at the right time matters
Have you ever slept eight full hours and still woken up feeling wrecked, then another day woken after six hours feeling sharp? The difference usually isn't how long you slept. It's which stage of sleep your alarm interrupted.
When your alarm pulls you out of deep N3 sleep, you get hit with sleep inertia: a foggy, slow, mentally impaired state where reaction time drops by a quarter or more and can take 15 to 30 minutes to clear. Wake at the natural end of a cycle, when you're already drifting in light sleep or REM, and the transition to alert is smooth. You can be clear-headed within a minute or two.
That's the entire idea behind cycle-aligned timing. By scheduling your sleep in whole 90-minute blocks, you aim your alarm at the gentle end of a cycle rather than the middle of your deepest sleep. For most adults, five cycles (about 7.5 hours) is the sweet spot, and a cycle-aligned 7.5 hours routinely feels better than a poorly-timed 8 hours that ends in deep sleep.
If you want the full picture, our guide to the 90-minute sleep cycle walks through what happens in each stage and why the pattern is asymmetric across the night.
Quick Reference
When should you go to bed?
If you'd rather not open the calculator, here's the quick math for common wake-up times, assuming an adult on five cycles (7.5 hours) with 15 minutes to fall asleep:
| Wake-up time | Bedtime (5 cycles, 7.5h) | Bedtime (6 cycles, 9h) |
|---|---|---|
| 6:00 AM | 10:15 PM | 8:45 PM |
| 6:30 AM | 10:45 PM | 9:15 PM |
| 7:00 AM | 11:15 PM | 9:45 PM |
| 7:30 AM | 11:45 PM | 10:15 PM |
| 8:00 AM | 12:15 AM | 10:45 PM |
These are starting points. Your ideal bedtime depends on your age, your personal cycle length, and how long you take to fall asleep, all of which the calculator above accounts for. The most important factor of all is consistency: going to bed and waking at roughly the same time every day, including weekends, keeps your body clock anchored so the whole system works better.
Dig deeper with our guides on what time you should go to bed and the best time to wake up.
The Method
How accurate is a sleep cycle calculator?
A sleep calculator is a planning tool, not a medical device, and it's honest to say so. It's built on well-established averages: a 90-minute cycle, a 15-minute time to fall asleep, and the National Sleep Foundation's age-based duration ranges. Real sleep varies from person to person and night to night, so treat the times as informed guidance rather than a precise prescription.
What the calculator gets right is the underlying structure: sleep genuinely runs in cycles, deep sleep genuinely clusters early in the night, and waking from deep sleep genuinely causes grogginess. Aligning your alarm to whole cycles is a sound strategy backed by decades of sleep research, even if your personal cycle runs a little shorter or longer than 90 minutes.
To make it more accurate for you specifically, two things help. First, if a wearable (such as an Oura ring, Whoop, or Apple Watch) reports your average cycle length, plug that into Advanced settings instead of the default. Second, use the calculator consistently for a couple of weeks and pay attention to how you feel on waking. Your own body is the best feedback loop, and small adjustments to your cycle length or fall-asleep time quickly dial it in.
More tools
Explore the full calculator suite
Built on the same science. One tool for every sleep question, naps, sleep debt, baby schedules, and more.
Frequently Asked Questions
How does the sleep calculator work?
The calculator uses the 90-minute sleep cycle model. A complete sleep cycle moves through light sleep (N1, N2), deep sleep (N3), and REM sleep, then repeats. Waking at the end of a cycle, when you're in lighter sleep, feels more refreshing than being jolted awake in deep sleep.
Enter when you want to wake up (or go to bed), pick your age band, and the calculator shows times that complete 3 to 6 full cycles. The recommended option for your age is highlighted.
How long is one sleep cycle?
The classic average is 90 minutes, but real cycles range roughly 70–120 minutes and tend to get longer through the night. Use the Advanced setting to adjust the cycle length (60–120 min) if you know yours runs shorter or longer than average.
Why do I still wake up tired after 8 hours of sleep?
The most common reasons are (1) waking in the middle of a deep-sleep stage, which causes sleep inertia, that groggy 15–30 min feeling, (2) inconsistent sleep timing throwing off your circadian rhythm, or (3) sleep that's fragmented by stress, light, or sleep disorders. Aligning your alarm to the end of a cycle helps with the first; consistent bed/wake times help with the second.
What time should I go to bed to wake up at 6 AM?
For a 6:00 AM wake-up, the cycle-aligned bedtimes (assuming 15 min to fall asleep) are approximately: 8:45 PM (6 cycles, 9h), 10:15 PM (5 cycles, 7.5h, recommended for most adults), 11:45 PM (4 cycles, 6h), or 1:15 AM (3 cycles, 4.5h). Use the calculator above for exact times tailored to your age.
How many hours of sleep do I actually need?
The National Sleep Foundation's consensus panel recommends 7–9 hours for adults 18–64 and 7–8 hours for 65+. Teens need 8–10 hours, school-age children 9–11, and infants up to 17 hours including naps. The "right" amount varies, but the calculator uses these evidence-based ranges to highlight a recommended option.
Is this sleep calculator accurate?
It uses well-established averages: a 90-minute cycle (with adjustable range), 15-minute sleep latency, and the NSF age-based duration ranges. Real cycles vary person-to-person and night-to-night, so treat the times as guidance, not a strict prescription. The more consistently you use them, the better feedback you'll get from your own body.
What is sleep inertia and how do I avoid it?
Sleep inertia is the groggy, mentally foggy state you sometimes feel right after waking. It's strongest when you're pulled out of deep (N3) sleep. To minimize it: wake at the end of a cycle (use this calculator), expose yourself to bright light within minutes of waking, and keep a consistent wake time even on weekends.
Should I sleep 6 cycles (9h) or 5 cycles (7.5h)?
For most adults 26–64, the sweet spot sits in the 7–9 hour range. If you regularly wake before your alarm, feel alert at 7.5h, and don't crash mid-afternoon, 5 cycles is plenty. If you wake naturally closer to 9h and feel best then, go with 6. The science says quality and consistency matter as much as duration.