The two formulas
There are two directions in the calculator. Both use the same planning model.
From wake-up time to bedtime
Bedtime = wake-up time − (number of cycles × cycle length) − time to fall asleep
If you need to be awake at 6:30 AM, assume five 90-minute cycles and allow 15 minutes to fall asleep, the calculation is:
- 5 × 90 minutes = 450 minutes of sleep
- 450 + 15 = 465 minutes before the alarm
- 6:30 AM minus 465 minutes = 10:45 PM
From bedtime to wake-up time
Wake-up time = bedtime + time to fall asleep + (number of cycles × cycle length)
The same assumptions work in reverse. A bedtime of 11:00 PM with five 90-minute cycles and 15 minutes of latency points to 6:45 AM.
Why show several results?
Sleep timing is not an exact science. Two people can have the same bedtime and wake up with different experiences. A single “perfect” time would create false precision.
Sandy therefore shows four cycle counts:
- 5 cycles: the recommended planning option when using a 90-minute model.
- 6 cycles: a longer night when your schedule allows it.
- 4 cycles: a shorter night that may fit a tighter schedule.
- 3 cycles: a short option for exceptional situations, not a target for most nights.
What the 15-minute latency means
“Sleep latency” is the time between getting into bed and falling asleep. Fifteen minutes is a common planning assumption, not a measurement of your body.
If you usually need longer, adjust the slider. If you fall asleep almost immediately, use a lower value. Avoid treating a short latency as a goal; feeling sleepy enough to fall asleep very quickly can sometimes be a reason to talk with a clinician, especially when it happens often.
What the model cannot do
- It cannot measure your actual sleep stages.
- It cannot know whether you will wake during the night.
- It cannot account for caffeine, alcohol, illness, stress or a bedroom environment.
- It cannot diagnose insomnia, sleep apnea or another sleep disorder.
- It cannot make a short night equal to a full night by choosing a different number of cycles.
Why the calculation stays in your browser
The calculator uses your device's clock and your selected inputs. It displays your device's IANA timezone and UTC offset, and calendar downloads include that timezone. It does not convert between departure and destination timezones; that belongs in a separate travel planner. The calculator does not need an account and does not send your times to a server.
How to use the result
- Choose the fixed time you cannot move.
- Adjust the latency and cycle length if the defaults do not fit your experience.
- Compare the options and choose one that gives you enough time in bed.
- Keep the plan for several nights before deciding whether it works.