What this site cannot know

Nothing you type or set here is sent to the site. It therefore has no way of knowing:

• how long your sleep cycles actually run, or how much they vary between nights;
• how long you take to fall asleep, tonight or in general;
• how much sleep you need, which differs by person and by stage of life;
• whether you slept badly last night, or for the last month;
• your age, your health, your medication, or anything else that bears on any of the above;
• whether you are shift working, jet lagged, unwell, or simply up late;
• whether the alarm you typed in is one you will actually honour.

What it has is two numbers you can see and change, and the ability to subtract. Every sleep table on this site is downstream of exactly that.

Why say so at this length

Because sites of this kind usually do the opposite. A confident number with no visible assumptions reads as a measurement, and people reasonably act on measurements. The arithmetic here is worth having; it is worth having as arithmetic.

What it does read

A few things, all of them inside your own browser, and none of them sent anywhere:

• your device's clock, on the pages that count from the present moment: the sleep-now page, the nap pages, the screen cutoff's "from now" line, the date the travel planner starts on, and the live times on the world clock;
• your device's time zone setting, which the world clock names above its list of cities;
• what it keeps in your browser's local storage: the theme, the clock format, the two sliders, the week's tally and the find-your-cycle notebook, each listed by name on the cookies and storage page, and the offline copy of the site's own pages if you asked for one;
• the settings a link to a calculator carries after the # sign, which the page reads and then takes out of its address before any other script runs, and which your browser does not send when it asks for a page.

None of that says anything about how you sleep. The exceptions are the week's tally and the notebook, which hold only what you typed into them, and they stay on your device. The length the notebook fits is arithmetic on those notes, not a measurement.