I'm not sure about absorption - I know some meds absorb better on an empty stomach. I would be concerned that taking the vitamins on an empty stomach might cause discomfort, and then a hard time sleeping. I know that I have had trouble with this in the past.
Good question - not sure what might be absorbed better at night, but I can tell you that when I take my B complex supplement or my SuperMom multi at night, they keep me up. Both give me a little burst of energy quite quickly.
Leeann, mama to 3*magic*kids: DD 1/03 DD 9/04 DS 8/06
Personally I think it's best to take the vitamins around the same times that you're eating, because they're basically a food supplement. I often end up taking vitamins after dinner though, because that's when I remember. I'm going to keep in mind that they may be boosting my energy, because I'm prone to staying up late!