Anyone know? Someone must know? Can't find answer anywhere.
The competition starts in the daytime. It takes time to shoot a television show, so it does not go as fast in real life as it does on TV. Usually the lesser know competitors go first (in the daytime), and the more well-known competitors compete toward the end (as in most competitions). Therefore, it is usually night-time by the time you see the more well-known competitors.
From the shows I've seen, where some of them are in very hot climates, I did notice that the entire show is done at night. True, the entire taping must be taking hours. They've got to reset the course and everything, I'm sure that takes time. But it occurs to me that perhaps they're doing this before dawn. Think about it, in places like Las Vegas and Texas, it's hot as hell after dusk but probably not so bad before dawn. So I'm guessing that they're doing the show at 4am, 5am, before dawn when it's not so hot yet, especially if humidity is a factor. Not to mention, those aerial shots seem like the city is dead, which wouldn't make sense if it was right after dusk, but would make a lot of sense if it was really early (before dawn). So I'm placing my money on a before-dawn time table.