I always used to exercise in the mornings, but I've started running in the evenings too now and it took me a while to get into the habit and be able to sleep afterwards, but I seem to be getting used to it. My other problem is that the longer I go before I work out the more likely I am to make excuses, but for other people it might be easier to make excuses not to get out of bed in the morning, it depends on the person. There's no "magic" time to exercise - if you can fit it in and it works for you, then go for it.
SW - 260, Original GW - 160 - achieved 21 March 2006, CW erm... I'll get back to you on that
4 sub-4 hour marathons! (PB 3:44:05)!