What’s the breakdown by gender? I’ve heard that women prefer it in the evening and men prefer it in the morning.
This, certainly, is the case with me and the missus. We compromise by doing it in the morning AND the evening. I love to compromise.
Well, I’m not fussy about the timing, but Morning is often the only time we both have the energy required so it just makes sence. Like many modern fulltime working couples we squeeze it in 
whenever we can.
You’ve gotta get up and have a shower etc soon anyway. Why not get sweaty now?