For the majority of people, the hurdle is getting exercise regardless of time of day. I think once you get past that, everything else comes down to personal preference and priorities.
I second that.
Started convict conditioning a month ago and I don't have a rigid schedule for training. But I will train every other day regardless of what happens on that day. This can mean I will do it at midnight but usually I do it earlier taking a break from work. It is a great way to channel procrastination into something useful.