I know I personally can't get started on things right away in the morning because of the psychic weight standups carry. I can't get focused because in the back of my mind I know I'm going to get interrupted soon for a meeting that mostly has little relevance to me.
Also, there's nothing that says that standups have to be in the morning. Most of the teams I've worked on actually had theirs either right after lunch or mid-afternoon. That accommodated those of us who were late-risers, it let the early-risers get some work in in the morning, and it meant that it'd often fall during food coma or when energy levels hit a mid-afternoon nadir.