I was watching something on the breakfast news a few months ago (I know I am sad) that said that caffiene, including that in soft drinks, acts very like sugar. At first you get the rush and feel more awake, but then that goes and you feel more tired than you did before. Apparently though, if you drink a lot of caffiene, its effects are reduced because your body gets used to it.
I'm not sure about this, sometimes I think I do notice something like this, other times - nothing. Maybe it depends on how tired you are to begin with?

