The fifteen minutes before you are asleep

The cycles only start once you are asleep, so a calculator that ignores the gap is telling you when to be unconscious rather than when to go to bed. Fifteen minutes is this site's default because it is a quarter hour, which keeps the arithmetic legible, and because no figure could be right for everybody; it is a starting point, not a norm.

It is also the number with the widest honest range. Someone who is asleep before the light is off and someone who lies there for a long while may both be reading this, and no page can tell which one it is. The slider runs from nothing at all to forty-five minutes for that reason.

Where it matters most

On a long night it barely moves the answer: fifteen minutes either way against seven and a half hours is a rounding error you would not notice. On a twenty-minute nap it is the dominant term — being ten minutes out about falling asleep is a fifty per cent error on the sleep itself.

So the honest reading is that the drop-off allowance is a small correction on the wake-time pages and the main event on the nap pages, which is the reverse of how much attention it usually gets.

Setting it

If you know roughly how long you lie there, put that number in. If you do not, the way to find out is to notice, over a few nights, rather than to accept a default from a website. Nothing here can measure it and nothing here pretends to.