Sleep is not one continuous state. It moves in cycles of light sleep, deep sleep and dreaming, and waking at the wrong point in that pattern is why an alarm can feel brutal after eight hours and easy after six. This page does not measure your sleep — nothing on a web page can — but it does the arithmetic that lines your alarm up with the gaps between cycles instead of the middle of one.

Using it in three steps

1. 1 Choose which end you know — the alarm you have set, or the time you are heading to bed.
2. 2 Adjust the fall-asleep minutes if fifteen is not your number.
3. 3 Take a time from the list. Later entries mean more cycles, not merely more minutes.

Why fifteen minutes of lying awake counts

The cycles start when you fall asleep, not when you switch the light off. If it takes you fifteen minutes to drift off, an alarm set six hours after bedtime lands three quarters of the way through the fourth cycle — the worst place to be pulled out of. That single quarter of an hour is the difference between the list this tool gives you and the naive "six hours from now", and it is why the field is there at all.

Ninety minutes is an average, not a promise

The ninety-minute figure is a population average, and yours may sit anywhere from seventy to a hundred and twenty minutes. Cycles also shorten and lengthen through the night, with deep sleep front-loaded and dreaming taking a larger share towards morning. Treat the times as candidates worth trying rather than as a schedule: if one of them consistently leaves you groggy, the next one along is probably closer to your own rhythm.