This article is from the Aerobics Fitness FAQ, by firstname.lastname@example.org (Robles) with numerous contributions by others.
As a general rule, if your habits are diurnal, exercise
in the early evening, when your metabolism is at its peak,
is more efficient. This varies widely, however, and you
really need to exercise at the time which "feels" best for
you. The best time to work out is when you *want* to, so
pick a time of day at which you find exercise enjoyable.