Mine depends on the weather. When it’s warm/hot, they do their flat-out snoozing in the early morning hours between midnight and sunrise. When it’s cool/cold, they take their snoozes in late morning sun. Some of them will do after-dark naps in the cold, some won’t.
Rain and or wind curtails extended snooze time. Sometimes, when the weather is about to change for the worse, I’ll see the whole bunch laying down at an “odd” time.
Generally speaking, if a horse gives a full body shake upon rising, he’s feeling fit as a fiddle. If he only shakes his head or doesn’t even do that, something may be amiss. Especially if he lays down again fairly shortly thereafter. (Caveat: this observation comes from outdoor horses; I don’t know if feeling-fine stalled horses will shake every time they rise or not.)