Ideally I exercise every day in the afternoon. At times I have exercised early in the morning, late at night, mid morning, ... all those work fine. The important thing for me is to do the cardio every day. When I miss one day, I start to feel bad, and it is harder to make myself do the cardio on the next day. When I do it every day, I don't argue with myself about maybe being lazy today. When I take two days off, I get so depressed that I might miss a whole week before I can turn around the downward spiral, so I try to do it every day.

A problem with late at night is getting to sleep and eating afterwards. It's not good to eat right before going to sleep, and the exercise usually makes me a little hyper afterwards.