Most
personal trainers do recommend working out in the morning, but it's
not the best option for everyone. Aside from the time constraints
imposed on people who work early in the morning, some people are so
sluggish that they won't get the workout they'd get at a different
time.
I've found that whatever a person will STICK with is his/her best
option, in all aspects of fitness. Studies have shown that morning
people are usually more energetic and more driven than non-morning
people, so they would naturally be more vigilant about exercise. I
don't think it's the morning itself that determines this, but a person's
tendency to be a morning person.
There are certainly benefits to exercising early in the day, getting
your metabolism revved up before you've had any big meals, but you
won't burn twice as many calories or anything, and if you're not a
morning person, you probably won't enjoy it and therefore won't keep
doing it.
If you do exercise later in the day, try to do it at least three
hours before you go to bed, so your body can wind down. Otherwise,
you might have trouble sleeping, which is totally counterproductive.
Some trainers even take into account the rise and fall of a person's
body temperature throughout the day, but I don't think you need to
get technical. Whatever fits into your life in a manner that you can
comfortably stick with is your best bet!