Tour de France - why does it start so late in the day?

The ~noon start makes viewing very convenient for this viewer (near Chicago) but it seems like it would cause the race to take part in the hottest part of the day. That seems to me like it would not be good for the riders. (My experience with marathons - heat takes a lot out of me, but I'm not getting the air flow that the riders get nor am I climbing mountains, not to mention the difference in condition between elite athletes and myself. )

Is it better for the riders to have time to prepare after waking before racing? Is the timing geared toward TV viewing hours in US and/or Europe? Does it relate to logistics?

Thanks!