What this site is not
Stated plainly and on its own page, because it is the single most important thing to understand about every number here.
This is a calculator
DuskDial multiplies an assumed sleep cycle length by a whole number, subtracts the result and a drop-off allowance from a time you gave it, and prints what comes out. That is the entire mechanism. Both assumptions appear on every page that uses them and both can be changed by you.
It is not medical advice
Nothing on this site is medical, clinical or health advice, and nothing here is a diagnosis or a treatment plan. No page claims that going to bed at any particular time will improve your health, your mood, your memory, your performance or your wellbeing. Those would be claims about a body, and nobody operating this site has examined yours.
It has measured nobody
The ninety-minute cycle is a convention drawn from laboratory averages, not a measurement of you, and real cycles vary between people and across a single night. The fifteen-minute drop-off allowance is a guess about an ordinary evening. A full account of what the site has no way of knowing is set out in the reference pages.
When to ask somebody else
If you regularly cannot fall asleep, wake repeatedly, sleep and are still exhausted, snore heavily or stop breathing in the night, or if tiredness is affecting your safety at work or on the road, that is a matter for a doctor. A bedtime calculator has nothing useful to say about any of it, and the fact that this one prints a confident-looking table does not change that.
The travel pages
The jet lag planner is clock arithmetic. It works out the difference between two time zones and what moving a bedtime by 30, 60 or 90 minutes a night would look like. It makes no claim about how quickly anybody adjusts to a new zone. It prints sunset and sunrise at the destination so the bedtimes can be read against them, and says nothing about what light, meals, medication or anything else would do.
The shift, clock-change and notebook pages
The shift rota planner fits whole cycles between the hours of a rota you give it, and the clock-change planner walks a bedtime across the hour the clocks go forward. Neither says which arrangement is better for anybody, and the only thing the shift planner marks is a sleep that will not fit between two shifts. The find-your-cycle notebook fits a cycle length to mornings you recorded yourself. That is a fit to your notes, not a measurement of your sleep, and it says nothing about your health.
The week's tally
The tally subtracts a target you chose from figures you typed in. It is not a measure of anything owed and it does not claim that a shortfall can be made up by sleeping longer later.
Do not rely on it for anything that matters
Use a real alarm. This site prints a time; it does not wake anybody up, and it is not a safety system. The terms set out the limits of our liability.