I don't know the scientific answer, but anytime would work. I prefer afternoon/evenings because when you wake up your body may not be ready for a run (depending on age, injuries, etc.). You may feel fine, other than sleepy, but you're at risk for injury greater because you're not fully with it yet (assuming you're not a morning person like me). See my body takes awhile to get ready so I go in the evenings after stretching so I'm fully alert and ready to do my cardio.
This post was edited by rspaid on Jan 7 2015 10:32am