Count backwards from the alarm and see which bedtimes land near the end of a sleep cycle.
The first Pillow Math spreadsheet did one job: it counted backwards from the time I had to be up. I still use that arithmetic, with the reminder that 90 minutes is an average and your night will not follow a ruler.
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 calculator
Start from your alarm and count back in sleep cycles. Here is the arithmetic I use, the studies the numbers come from, and how much they can be trusted.
Read more: What time should I go to bed?