A sleep calculator built on 90-minute sleep cycles
This sleep calculator answers two everyday questions: “what time should I go to bed to wake up at 7?” and “if I go to sleep now, when should I set my alarm?”. It counts back or forward in 90-minute sleep cycles, adds the time it usually takes you to fall asleep, and shows every option from a short night to a full one — with the best fit for your age highlighted.
Waking at the end of a cycle, during lighter sleep, tends to feel easier than being dragged out of deep sleep halfway through one. The calculator also plans naps and shows how many hours of sleep the National Sleep Foundation and the American Academy of Sleep Medicine recommend for each age group.
How to use the sleep calculator
- Choose a mode: Wake up at… to get bedtimes, Go to bed at… to get wake-up times, or Sleep now to plan from the current time.
- Enter the time, and adjust time to fall asleep if you usually drift off faster or slower than 15 minutes.
- Pick your age group so the calculator can mark which options give enough sleep.
- Use the highlighted time, or pick another option card — each one shows the number of cycles and hours of sleep.
- For a daytime rest, set the nap start time to see when to wake from a 20-minute power nap or a 90-minute full-cycle nap.
Sleep Calculator features
Three planning modes
Work backwards from an alarm, forwards from a bedtime, or from right now — the “now” mode refreshes itself every 30 seconds.
Age-aware recommendations
Each option is labelled as too short, within the recommended range for your age group, or longer than needed.
Adjustable sleep onset
Set your own time to fall asleep from 0 to 60 minutes instead of a fixed 14 or 15.
Nap planner
Shows wake-up times for a power nap and a full-cycle nap from any start time.
Local time format
Times appear in 12-hour or 24-hour style to match your device settings.
How sleep cycles work
Through the night you move through repeating cycles of non-REM and REM (rapid eye movement) sleep. A full cycle averages about 90 minutes, although it can range from roughly 70 to 120 minutes, and most adults complete four to six cycles a night. Early cycles contain more deep, slow-wave sleep, which helps physical recovery; later cycles contain longer periods of REM sleep, which is linked to memory and emotional processing.
The formula: bedtime = wake-up time − (cycles × 90 minutes) − time to fall asleep. To wake at 7:00 AM after five cycles with 15 minutes to drift off: 7:00 − 7 h 30 min − 15 min = 11:15 PM. Six cycles moves bedtime to 9:45 PM.
| Stage | What happens |
|---|---|
| N1 (light) | Transition from wakefulness; lasts only a few minutes |
| N2 (light) | Heart rate and temperature drop; makes up about half of the night |
| N3 (deep) | Slow-wave sleep; hardest to wake from; most common early in the night |
| REM | Vivid dreams, active brain, relaxed muscles; longer towards morning |
How much sleep do you need?
Both major guidelines agree that most adults need at least seven hours a night: the National Sleep Foundation recommends 7–9 hours for adults aged 18–64 and 7–8 hours from 65, and the American Academy of Sleep Medicine and Sleep Research Society advise seven or more hours for adults. Teenagers need 8–10 hours and school-age children 9–11 (NSF) or 9–12 (AASM).
For most adults that makes five cycles (7.5 hours) or six cycles (9 hours) the sweet spot. Four cycles is only six hours — fine for an occasional early flight, but regularly sleeping less than seven hours is linked with poorer concentration, mood and long-term health. Babies and toddlers have shorter sleep cycles than adults, so the cycle timings are designed for older children and adults.
Once you have picked a time, set it on the online alarm clock, or check how long you actually slept with the time duration calculator. These suggestions are general guidance, not medical advice.
Napping and better-sleep tips
Good sleep depends on more than timing. These habits make any bedtime work better:
- Power nap: 10–20 minutes of sleep keeps you in light sleep, so you wake up refreshed rather than groggy.
- Full-cycle nap: about 90 minutes lets you complete a cycle, including deep and REM sleep. Naps of 30–60 minutes are the most likely to leave you groggy.
- Timing: nap in the early afternoon. Late naps make it harder to fall asleep at night.
- Consistency beats precision: going to bed and getting up at the same times every day, weekends included, matters more than hitting an exact cycle boundary.
- Wind down: keep the bedroom dark, quiet and cool, and avoid caffeine in the afternoon and heavy meals or alcohol late in the evening.
Sleep Calculator FAQ
What time should I go to bed if I wake up at 7 AM?
For five 90-minute cycles plus 15 minutes to fall asleep, go to bed at 11:15 PM. For six cycles, aim for 9:45 PM. Four cycles (12:45 AM) gives only six hours, which is too little for most adults on a regular basis.
Is it better to sleep 6 hours or 7.5 hours?
Seven and a half hours, five full cycles, is better for most adults and meets the seven-hour minimum that sleep experts recommend. Six hours is four cycles and leaves most people short of sleep if it becomes a habit.
How accurate is a sleep cycle calculator?
It uses the 90-minute average, but real cycles vary between people and through the night, and the time it takes you to fall asleep changes too. Use the times as a starting point and adjust by 15–30 minutes based on how you feel when you wake.
How long should a power nap be?
About 10–20 minutes of actual sleep. Add the time it takes you to fall asleep when you set the alarm — the nap planner does this automatically using your “time to fall asleep” setting.
Why do I feel tired even after 8 hours of sleep?
You may have woken from deep sleep, slept poorly because of noise, light, alcohol or caffeine, or have a sleep disorder such as sleep apnoea. If tiredness continues despite enough sleep, speak to a doctor.
How long should it take to fall asleep?
For most adults, about 10 to 20 minutes. Falling asleep within a minute or two of lying down every night can be a sign that you are not getting enough sleep.