Sleep
Calculate optimal sleep and wake times
These calculators provide general information only and are not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.
How to use Sleep
- 1Pick the time you plan to fall asleep, or the time you need to wake up.
- 2Set how long you usually take to fall asleep, most people need 10 to 20 minutes, and the tool subtracts it.
- 3Read the suggested times, all aligned to the end of complete sleep cycles.
How sleep cycles are calculated
Wake time = bedtime + time to fall asleep + cycles × 90 minutes (4–6 cycles)Sleep unfolds in cycles of roughly 90 minutes, each containing light sleep (N1 and N2), deep sleep (N3) and rapid eye movement. Deep sleep dominates the first half of the night and handles physical repair and growth hormone release, while REM increases later and supports memory consolidation and emotional regulation. Waking at the end of a cycle, close to the surface, leaves you clear-headed, whereas being woken from deep sleep produces heavy sleep inertia and sluggishness.
So the calculator works from the moment you fall asleep, after subtracting the time it takes you to drift off, and steps forward in 90-minute blocks, listing the wake times for four, five and six cycles, which is 6, 7.5 and 9 hours. One caveat: 90 minutes is a population average, individual cycles run anywhere from 70 to 120 minutes and shorten slightly with age. Treat this as a practical scheduling aid rather than a precise read on your body clock.
| Cycles | Sleep | Typical use | Suits |
|---|---|---|---|
| 4 | 6 hours | Short-term, emergencies | Not for the long run |
| 5 | 7.5 hours | Everyday for most adults | Recommended |
| 6 | 9 hours | Recovery, after hard training | Teenagers or catching up |
Cycle counts and total sleep
Frequently asked questions
Do I really need eight hours?
Not necessarily. Eight is a round number that is easy to remember; real needs vary, and most adults fall somewhere between seven and nine hours. What matters more is completing whole cycles and how you feel on waking. If 7.5 hours leaves you sharp, there is no reason to force eight. The test is whether you feel sleepy during the day and struggle to get up.
Why am I still tired after a long night?
Often because you woke from deep sleep, or because quality was poor. Frequent awakenings, snoring with pauses in breathing, alcohol before bed, and a bedroom that is too bright or warm all reduce deep and REM sleep. If you consistently wake unrefreshed despite adequate hours, especially with loud snoring and morning headaches, ask about a sleep study.
How long should a nap be?
Ten to twenty minutes is ideal: it restores alertness without reaching deep sleep, so you wake clear-headed. Thirty to sixty minutes tends to end in deep sleep and produces noticeable grogginess. If you want a full cycle, allow 90 minutes. It also helps to nap before about 3 pm so night-time sleep is not affected.
Does sleeping in at the weekend help?
It repays part of the debt but cannot fully offset chronic short sleep, and it shifts your body clock, making Monday harder, the so-called social jetlag. Keeping bedtime and wake time within about an hour every day works better. If you are genuinely short, going to bed an hour earlier beats sleeping three hours later.