Tell it when you have to be up. It counts backwards in ninety-minute rounds and tells you when to get into bed, which is a more answerable question than how many hours you should have had.
The reasoning
Sleep is not one flat state you sink into and climb out of. You move through it in rounds, from light sleep into deep slow-wave sleep and then into REM, the stage where most vivid dreaming happens, before surfacing close to waking and starting again. A full round takes somewhere around ninety minutes, and a normal night is four to six of them.
How rough you feel on waking depends less on how long you slept than on where in the round the alarm caught you. Being pulled out of deep slow-wave sleep produces the heavy, stupid feeling researchers call sleep inertia, and it can hang around for a while. Surfacing near the end of a round, when sleep is already light, tends to feel considerably less like being hit with a shovel.
That is the entire trick this calculator is doing. It is not optimising your sleep. It is trying to put your alarm somewhere less brutal.
Here is the part most calculators leave out. Ninety minutes is a population average, and real cycles run roughly seventy to a hundred and twenty minutes depending on the person. They also change shape across the night: the early rounds carry more deep sleep, the later ones carry more REM, so they are not even the same length as each other in one night.
So treat every time on that grid as roughly correct rather than precisely correct. If a suggested time feels wrong for you by twenty minutes or so, you are probably right and the average is probably wrong.
Nobody falls asleep the instant they lie down. Healthy sleepers usually take something in the range of ten to twenty minutes, which is why the calculator adds that time before it starts counting rounds. It is also why we let you change the number: if you know you take forty minutes, using fifteen throws every single result out by nearly half an hour.
For most adults the usual recommendation is seven to nine hours, which is five to six of those rounds. Teenagers need more, older adults often sleep in shorter and more broken stretches, and a minority of people genuinely sit outside the range in both directions.
Worth saying plainly: hitting a clean cycle boundary on five hours does not make five hours enough. The calculator can tell you when to set the alarm. It cannot buy you sleep you did not take.
Children and teenagers need meaningfully more than adults do, and a teenager running on an adult's seven hours is underslept rather than fine. If you are working out a bedtime for someone who is not a grown adult, look up the guidance for their age rather than using this page's assumptions.
Going to bed and getting up at roughly the same time every day, including at weekends, does more for how you feel than any arithmetic on this page. So does daylight in the morning, and not lying in bed awake for an hour fighting it.
None of this is medical advice, and we are still not doctors. If you genuinely cannot sleep, night after night, no calculator is the answer and someone qualified is.
Most adults are told to aim for seven to nine hours, which is five or six of those ninety-minute rounds. That range is an average across a lot of people, so some of us genuinely need more and a few genuinely need less, and neither makes you broken. Teenagers need more than adults do. Older adults often sleep in shorter, more broken stretches, and that is normal rather than a failure.
What the calculator can do is stop your alarm landing in the middle of deep sleep, which is most of why mornings feel like being hit with a shovel. What it cannot do is turn five hours into enough. Landing neatly on a cycle boundary at 2am is still 2am.
Short answers, including to the ones we would rather dodge.
Roughly right, and not precisely right. Ninety minutes is an average across a lot of people; real sleep cycles run somewhere around seventy to a hundred and twenty minutes and change shape across the night.
So use the times as a target to aim near rather than a schedule to hit. If one feels wrong for you by twenty minutes, trust yourself over the average.
Because nobody falls asleep the moment they lie down. The calculator assumes a stretch of lying there first, and you can change that number, which matters more than people expect: if you actually take forty minutes and the page assumes fifteen, every result is out by nearly half an hour.
Then the arithmetic is off, and that is fine. Waking briefly between cycles is normal and most people do it without remembering. It only becomes a problem when you start lying there doing sums about how much sleep you have left, which this page can unfortunately encourage.
If that is you, close the tab and stop counting. Working out that you have four hours and eleven minutes remaining has never once helped anybody fall back asleep.
No. Not doctors, not sleep scientists, not therapists, not qualified in any of this. We are people who could not sleep, got interested in why, and started building small things to try on ourselves.
So read everything here as somebody's experiment rather than as advice. Where we mention research we have tried to describe it honestly, including the parts where the evidence is thin, but we are reading it as curious amateurs and you should treat it that way too.
If you genuinely cannot sleep, night after night, for weeks, that is worth taking to someone qualified. Insomnia is treatable and the treatment that works best is not a website.
Honestly, it depends on the thing and on you. Some of what is here leans on techniques with real evidence behind them, some of it is closer to a nice idea that helps because it gives your head something dull to do, and we try to say which is which on each page.
What we will not do is tell you something is clinically proven when it is not. A sleep site that oversells itself is just another thing keeping you up.
There are no accounts, so there is nothing to log into and nothing tying any of this to you. Your settings, the voice you picked, how long you say you take to fall asleep, live in your own browser and never leave it.
The one exception is deliberate: if you queue a sound or leave an email address, you have sent us that on purpose, and the queue is public by design so everybody sees the same list.
There is no catch and no paywall, mostly because there is nothing expensive to pay for. The sounds are generated in your browser rather than streamed, so there are no files to host and no licences to buy.
If that changes we will say so plainly rather than quietly putting a wall in front of something that used to be free.
Not medical advice. If you really can’t sleep, go see someone real. We just have the rain sounds.
Have a nice sleep, bro.