How does the sleep calculator work?
Sleep isn't one long, even stretch. During the night your brain moves through repeating sleep cycles, each lasting roughly 90 minutes. Each cycle takes you through light sleep, deep sleep, and REM sleep — the stage tied to dreaming and memory.
Waking up in the middle of a cycle — especially out of deep sleep — is what produces that heavy, disoriented feeling known as sleep inertia. Waking at the end of a cycle, during lighter sleep, feels dramatically easier.
This calculator counts back from your wake-up time in 90-minute cycles and adds about 14 minutes for the average time it takes to fall asleep. A good night's sleep consists of 5–6 complete cycles (about 7.5–9 hours).
What time should I go to bed?
Work backwards from when you need to get up. If your alarm is at 7:00 AM, the calculator recommends going to bed around 9:45 PM or 11:15 PM. If those feel too early, the 4-cycle option (about 6¼ hours of sleep) still lets you wake between cycles — but you'll function noticeably better with 5 or 6 cycles most nights.
How much sleep do I need?
| Age group | Recommended sleep |
|---|---|
| Newborns (0–3 months) | 14–17 hours |
| Infants (4–11 months) | 12–15 hours |
| Toddlers (1–2 years) | 11–14 hours |
| Preschoolers (3–5 years) | 10–13 hours |
| School children (6–13 years) | 9–11 hours |
| Teenagers (14–17 years) | 8–10 hours |
| Adults (18–64 years) | 7–9 hours |
| Older adults (65+) | 7–8 hours |
Source: National Sleep Foundation sleep duration recommendations.
Frequently asked questions
Should I fall asleep faster or slower than 14 minutes?
14 minutes is just an average. If you typically fall asleep in 5 minutes, your cycles simply start earlier and you'll get slightly more sleep. If it takes you 30 minutes or more on a regular basis, that's a sign to look at your bedtime habits — caffeine, screens, or an irregular schedule are common culprits.
Why do I still wake up tired after 8 hours?
Quantity isn't everything — timing matters too. Eight hours that end in the middle of a deep-sleep phase can feel worse than 7.5 hours that end at a cycle boundary. Try shifting your bedtime by 15–20 minutes for a week and compare how you feel.
Do sleep cycles really last exactly 90 minutes?
90 minutes is a widely used average, but individual cycles range from about 70 to 120 minutes, and cycle length varies across the night. Treat the calculator's times as strong starting points, then fine-tune based on how you feel when the alarm goes off.
What about naps?
The same cycle logic applies. A 20-minute power nap keeps you in light sleep so you wake easily. A full 90-minute nap lets you complete one entire cycle, including deep sleep and REM. Avoid mid-length naps (~45–60 minutes), which tend to end in deep sleep and leave you groggy.