Clearly, doctors can't be at the hospital 24 hours a day, 7 days a week, so for at least some of the time, their patients have to be cared for by somebody else. Different hospitals handle this in different ways. At Pennsylvania Hospital, there is a night float system. Most of the interns leave the hospital at night and sign their patients out to a single night float intern who works from about 5pm to 7am. The good thing is that most interns actually get to leave the hospital at night and are refreshed the next day to do work. The bad thing is that the night float intern is taking care of probably about 80-100 patients a night, and since you get signout on so many patients, it's not that detailed and you really only remember the really sick patients or make a note of specific things you have to do for certain patients. That means that night float doesn't know most of the patients very well at all - they just handle critical issues at night. Most patients are sleeping at night, and there's not much going on, so it's manageable, but if there are quite a few sick patients that night, it can get very busy and complicated. The good thing is that there is a resident or two for backup if things do get ridiculously busy.
Some other hospitals do it differently. They have teams that round together, and at night, 3 interns will sign out to one other intern for the night. The good thing is that that since they all round together, that intern staying overnight is supposed to know all the patients relatively well. The bad thing is that one out of every 4th intern stays overnight, making 25% of the interns extremely tired and miserable the next day (since usually they get very little to no sleep) and also decreasing the work force by 25% the next day since they have to leave by noon.
It's hard to say which is the better option. It's bad to have so many interns tired working the next day as studies show that the more tired doctors are, the more mistakes
can be made (not that they are common by any means). This is an argument for the first system, like at Pennsylvania Hospital. However, newer studies are starting to show that the more times care is transferred between different doctors, especially doctors who are not super familiar with the patients to begin with, the more mistakes
can be made (again, not that they are common). This would be an argument for the second system.
In fact, there are rumors of new legislation in the works that forces interns to take a nap (!) in the middle of call for at least 3-4 hours. During this time, they would have to somehow sign off their patients to someone else, and then get them back at the end of their nap. This is all part of ongoing legislation reform about duty work hours (a whole other post). However, the problems with transfer of care may be an argument against doing this sort of thing. Until hospitals have more money to hire more helpers (e.g. RN's, NP's, etc) that can at least do part of the work of interns, I guess we'll have to make do.