How to use the online timer
Twenty minutes lying down at the end of the day, with the lights off and nothing to look at, is a way of putting a boundary between the evening and sleep. It suits a household that has been awake since before six, or anyone whose day ended at a laptop and whose head is still running through tomorrow's list at eleven at night.
Set the twenty minutes and then forget about it, so there is no reason to check anything. Lie down, let the breathing slow, and work through the body from the feet upwards if you want something to hold attention. Keep this page open on a device across the room rather than in bed with you, since the count needs the tab and you do not need the screen.
Falling asleep before the end is a perfectly reasonable outcome and not a failure of the exercise. If the same twenty minutes leaves you more awake than before, it is worth trying it earlier in the evening rather than as the last thing. Screens in the hour before bed work against all of this, which is a dull point that remains true anyway.
Common questions
What if I fall asleep before the end?
That is a perfectly reasonable outcome and not a failure. Set the count on a device across the room so nothing needs dealing with afterwards, and let the page finish on its own.
What if it leaves me more awake?
Try it earlier in the evening rather than as the last thing before bed. Screens in the hour beforehand work against it too, which is a dull point that remains true regardless.