My five-year-old son is similar to some of what I read here. He is a night owl (so am I in general, even though I'm up early today). He surprises us all the time by suddenly falling asleep, when he seemed to be very active just the minute before.
In our case we have tended to let him go and go until crashing, but now that he's in school again are having to adjust that on school nights. The new rule is "in bed by 10", though he has a flashlight and is allowed to read in the dark / under the covers without being "caught". And I have to say that we're not great about enforcing the 10 p.m. cutoff either...
I have to say that being able to go until one crashes seems like a useful skill to me. I don't want to discourage him from being driven, in any way. I always figure that if he's doing something useful, it doesn't matter that it's at midnight.