I was out this past Saturday as a designated driver (DD) until about 2:45 AM. This is way past my bed time. An interesting question that came up during this event, and even before, when I knew that it would happen that I would be out that late is, what role does the DD have in determining the course of the night? If the DD wants to go home at 11 PM does the group go home?
Ideally this is a group decision. Everyone would be on the same page. But as groups involved with alcohol often do, a subset of the group becomes interested in remote prospects for future entertainment, and everyone else follows along. However, in the rare circumstance that you are in a DD rotation with like minded friends (as I was in grad school) remember that what goes around comes around.
I do not have an answer, but I do have a few words from the DDs among us: we don't want to stay out as late as you, and we aren't having as much fun when we are out that late.