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.
This is part of the misc.fitness.aerobics Frequently Asked Questions page (FAQ).
If you have any suggestions, corrections, or questions about this page,
the comment page to let us know!