Sleep Calculator
Sleep runs in roughly 90-minute cycles. Waking at the end of a cycle — instead of in the middle of deep sleep — helps you feel more refreshed. Choose a mode below and get your best times instantly.
How the sleep calculator works
The calculator counts backward (or forward) in 90-minute steps and adds an average 15 minutes to fall asleep. In wake-up mode it shows the bedtimes that line up with the end of 4, 5, or 6 full cycles before your alarm. In sleep now mode it shows the alarm times that land at the end of a cycle if you go to bed right now. The 6-cycle option (about 9 hours) is highlighted as the fullest night; 5 cycles (7.5 hours) is the sweet spot for most adults.
What is a sleep cycle?
During the night your brain moves through stages of light sleep, deep sleep, and REM. One full pass takes about 90 minutes, and a healthy night is usually 5 to 6 cycles. If your alarm goes off in the middle of deep sleep, you wake up groggy (a state called sleep inertia). If it lines up with the lighter sleep at the end of a cycle, you tend to feel clearer and more rested — even on the same total hours.
Why 90 minutes?
Researchers describe the night as a repeating ultradian rhythm averaging around 90 minutes per cycle. The first cycles of the night are heavier on deep sleep, while later cycles contain more REM. Because the cycle length isn't identical for everyone, the 90-minute figure is a practical average rather than a precise law — which is why this tool gives you several options instead of one single "correct" time.
Bedtime to wake-up cheat sheet
Common targets, using 6 cycles (≈9 h) and 5 cycles (≈7.5 h), plus 15 minutes to fall asleep:
| Wake up at | Bedtime (6 cycles) | Bedtime (5 cycles) |
|---|---|---|
| 5:00 AM | 7:45 PM | 9:15 PM |
| 6:00 AM | 8:45 PM | 10:15 PM |
| 6:30 AM | 9:15 PM | 10:45 PM |
| 7:00 AM | 9:45 PM | 11:15 PM |
| 7:30 AM | 10:15 PM | 11:45 PM |
| 8:00 AM | 10:45 PM | 12:15 AM |
Wake-up time pages: 5 AM · 6 AM · 7 AM · 8 AM
Bedtime pages: go to bed at 10 PM · go to bed at 11 PM
More tools: if I go to sleep now · nap calculator
Recommended sleep by age
| Age group | Recommended sleep |
|---|---|
| Newborns (0–3 mo) | 14–17 hours |
| Infants (4–11 mo) | 12–15 hours |
| Toddlers (1–2 yr) | 11–14 hours |
| Preschool (3–5 yr) | 10–13 hours |
| School age (6–13 yr) | 9–11 hours |
| Teens (14–17 yr) | 8–10 hours |
| Adults (18–64 yr) | 7–9 hours |
| Older adults (65+) | 7–8 hours |
General guidance only, not medical advice. Sleep needs differ between individuals.
Power naps and the nap calculator
Naps follow the same cycle logic. A 10–20 minute power nap keeps you in light sleep and boosts alertness without grogginess. A 90-minute nap completes one full cycle, including REM, and can aid memory and creativity. Avoid the in-between 30–60 minute window — that's when you're most likely to wake from deep sleep and feel worse than before. Try the nap calculator to see the exact alarm time for each nap length starting from now.
| Nap length | Best for | Wake-up feel |
|---|---|---|
| 10–20 min | Quick alertness boost | Refreshed, no grogginess |
| 30 min | Generally avoid | Some grogginess |
| 60 min | Memory of facts/places | Possible deep-sleep grogginess |
| 90 min | Full cycle, creativity, REM | Refreshed |
Tips to fall asleep faster
- Keep a consistent bedtime and wake time, even on weekends.
- Dim the lights and avoid bright screens for an hour before bed.
- Keep the room cool, dark, and quiet.
- Avoid caffeine in the afternoon and heavy meals late at night.
- If you can't sleep after 20 minutes, get up and do something calm, then try again.
Frequently asked questions
What time should I go to bed to wake up at 6 AM?
To wake at 6:00 AM, aim to fall asleep around 8:45 PM (6 cycles) or 10:15 PM (5 cycles). See the wake up at 6 AM page.
If I go to sleep now, when should I wake up?
Use the "If I go to sleep now" mode above, or the sleep now page, which adds 15 minutes to fall asleep and then counts full 90-minute cycles forward.
Is the 90-minute cycle exact?
It's an average. Cycles range from about 70 to 110 minutes and shift through the night, so treat the results as a helpful guide rather than a strict rule.