Pick the time you have to be up. The calculator counts back in sleep cycles and gives you the bedtimes that end one, so the alarm lands on a light patch instead of deep sleep.
My phone looked like this for most of my twenties. Twenty-three alarms, two minutes apart, and I slept through the lot.
Guides to pillows, built from the manufacturers' own specifications and the arithmetic this site runs on. Nobody here has slept on these.
Sleep calculator
Is a sleep calculator accurate? The arithmetic is exact, but it rests on a 90 minute average. Where it holds, where it drifts and how I tune mine to fit me.
Read more: Is a sleep calculator accurate? The math is exact, the 90 minutes is not
Sleep schedule
How I improve sleep hygiene with light, a fixed wake time and a caffeine clock, the studies behind each habit, and the popular tricks that did nothing for me.
Read more: How to improve sleep hygiene, from the shift years when I got it all wrong
Sleep science
The 90 minute sleep cycle is an average. Yours may run 70 to 110 minutes. Here is where the number comes from and what the sleep calculator assumes.
Read more: The 90 minute sleep cycle is an average, and yours may be 70 or 110I spent my twenties on late shifts and slept through alarms for years. A spreadsheet that counted sleep cycles back from my wake time fixed it, and this site is that spreadsheet with the working shown. Waffles, the cat, wakes me at 5:40 regardless.