Ann.
My understanding of the night float system is fairly primitive, but I have spoken with some residents about it.
Generally, night floats are set up in the following way:
Take an Internal Medicine Residency that has 13 residents in each of the PGY-2 and 3 years. 26 spots total.
Each resident would serve as Senior Resident on Night Float for 2 weeks out of the year. Thus, you'll have 26 residents times 2 weeks each, and that covers the whole year.
That means that, when there is a night floater, the on call team only admits to, say, 10pm. It's a short call. That team takes care of their admits and goes home when work is done (sometimes midnight or 1am). Then the floater comes in and admits all the patients that come in after 10pm. The floater works until 7am, checks out the patients to the next days on-call team, and the floater goes home and sleeps. (Generally, they are excused from next day duty. Including lectures and stuff).
Thus, if you work 2 straight weeks of what is, essentially, a night shift, you'll have the other 50 weeks with no overnight responsibilities. Pretty cool, eh?
I understand that things often work different for the intern on call, and that every residency that has a night float system for upper level residents doesn't necessarily include interns in that system. Regardless, they are usually responsible for the in house patients that have been checked out to them. So, if someone needs Tylenol at 3am, the intern's getting the call. This is changing slowly, as well, and Intern Night Float is coming into the system pretty rapidly.
Hope that explanation helps you. If I'm wrong, please correct me, for anyone else who might know a little more.