Based on 90-minute sleep cycles. Choose your wake-up time or bedtime and get science-backed sleep and wake times — free, instant, and private.
A sleep calculator helps you work out the ideal time to fall asleep or wake up, based on your body's natural 90-minute sleep cycles, so you wake up at the end of a cycle rather than in the middle of deep sleep — the difference between feeling refreshed and feeling groggy.
Sleep happens in repeating cycles of roughly 90 minutes, moving through light sleep, deep sleep, and REM sleep. Waking up in the middle of a deep sleep phase is what causes that heavy, disoriented "sleep inertia" feeling — even after 7 or 8 hours in bed. Waking up at the end of a full cycle, on the other hand, tends to feel far more natural.
This calculator works backward or forward in blocks of 90 minutes from your chosen wake-up time or bedtime, and adds a 15-minute buffer for the average time it takes to actually fall asleep.
| Age Group | Recommended Sleep |
|---|---|
| Teenagers (14–17) | 8 – 10 hours |
| Young Adults (18–25) | 7 – 9 hours |
| Adults (26–64) | 7 – 9 hours |
| Older Adults (65+) | 7 – 8 hours |
These ranges are guidelines from sleep health organizations in the USA, UK, and Canada. Individual needs can vary — some people function well on 7 hours, while others genuinely need closer to 9.
💡 Tip: Rather than aiming for a round number like "8 hours," aim for a full number of 90-minute cycles (5 or 6 cycles = 7.5 or 9 hours). This is why the times above often land on slightly unusual numbers rather than exact hours.
Two people can both sleep for 8 hours and wake up feeling completely different — one refreshed, one exhausted. The difference usually comes down to where in the sleep cycle they woke up, how consistent their sleep schedule is night to night, and how much screen light and caffeine they had before bed.
Most adults aged 18 to 64 need 7 to 9 hours of sleep per night. Teenagers need 8 to 10 hours, and older adults over 65 typically need 7 to 8 hours, though individual needs can vary.
To complete 5 full sleep cycles (7.5 hours), you should fall asleep by around 10:30pm the night before — meaning you'd want to be in bed by roughly 10:15pm to allow time to fall asleep.
Yes. Waking up at the end of a 90-minute cycle, rather than in the middle of a deep sleep phase, generally leads to feeling more alert and less groggy, even if the total sleep time is similar.
This often happens when you wake up mid-cycle during deep sleep. Sleep quality, consistency, and waking at the right point in your cycle all matter as much as the total number of hours.
For short naps, aim for either 20 minutes (light rest, no grogginess) or a full 90-minute cycle (deeper rest, low grogginess). Naps of 45–60 minutes tend to leave people feeling groggy since they interrupt deep sleep.