For me, it is based on the forecast. If it is going to be windy, I will ride earlier before the winds pick up. If it is going to be hot/humid, I'll ride as early as I can muster.

I get up at 5 AM and ride 10 miles most mornings. I'd rather sleep in, but that's the only time I have to get some riding in every day, since I'm at work from 7:30 to 5. Plus it's a lot cooler at that time of day than in the late afternoon or evening. I did have a retired friend comment on one of my rides about how early I was riding.