Where the ninety minutes came from

In 1957 William Dement and Nathaniel Kleitman published a study of sleeping subjects wired to an electroencephalograph, describing a pattern that repeated through the night: periods of rapid eye movement returning again and again until morning. That recurring interval is the ancestor of every ninety on every page of this kind, including this one.

Two things about it are worth holding on to. It is an average, not a fixed length: a standard medical reference gives the cycle as averaging ninety minutes, and a 2023 review of sleep in recovery after stroke, in passing, puts the average first cycle of the night at about 70 to 100 minutes and later ones at about 90 to 120, so the interval runs shorter early in the night and longer later. And it describes groups of sleepers studied in laboratories, which is not the same kind of statement as a number about you.

What a calculator does with it

It takes the average, calls it ninety for every cycle of the night, multiplies by three, four, five or six, and subtracts. That is not wrong so much as extremely simplified, and the simplification is invisible wherever a calculator prints the result and not the assumption.

This site prints the assumption, and puts it on a slider from sixty to a hundred and twenty minutes. Ninety is where the slider starts, not where the truth is. If you have reason to think your own interval runs longer, move it and watch every time on the page move with it: at a hundred minutes rather than ninety, five cycles is fifty minutes further from the alarm.

Why the site still uses it

Because a shared convention that everybody can see is more useful than a precise number nobody has. The alternative to an openly stated ninety is not a measurement of your sleep; it is a different arbitrary number, stated less openly.

The companion assumption, and the one with the widest honest range, is the time it takes to fall asleep.