I have been to many barns in the US that have dirt floors and have yet to see a horse laying down during the day.
This is a horse on concrete floored stable, she came in, to be out of the flies, ate hay and went to bed. This was mid morning.
In the UK most people bed far more deeply and up to the door, which is rarely done in the US.
The last two are of young horses in the loose shed, (The first had just been castrated and the dog would not leave him until he was standing) These are on a dirt floor but if it was not bedded deeply they would not lie down very much at all.