To answer Qhamp's question,
Some schools require overnight call for medical students. Others don't. My school does, and it wasn't until I was well into my 3rd year (and experienced at staying up all night) that I learned there are some schools where students don't take overnight call.
My call schedule last year: Internal medicine overnight call was every 5th night, then you had to stay for the whole day after. Pediatrics was every 4th night, and again you had to stay the whole day after. It wasn't as bad as it sounds though, becuase there were 3-4 students per team, and we were assigned newly admitted patients in a rotating fashion. Most nights, we got 2-3 hours of sleep each. Some nights, though, it was busy enough that we never made it to bed. The primary duty of the student on these services was to write the H&P (=History and Physical, a document that each patient admitted must have on the chart, detaling the symptoms and physical exam findings, plus the plan for treating the patient). The residents we took call with also had to write a note on each patient, plus take care of whatever came up with patients already in the hospital (maybe they need tylenol, or maybe the have a heart attack in the middle of the night. All kinds of things must be addressed) Students were always welcome to tag along with a resident and see what was going on (but not required to). So generally we would go only if something sounded cool, or if there was a procedure that needed to be done.
Psych had no overnight call. We had to go in on one weekend day per month to write H&P on patients who had been admitted overnight, but when we finished that, we went home
OB/gyn had overnight call every 4th night on the OB month, but we got to go home after call was done. (Unless it was a day we had a lecture, then we had to go to the lecuture and then go home). OB call was much busier, much less sleep (often none), and when you did sleep you had to rotate and sleep one person at a time), and occured in an evaluation area (kind of like an ER) we took the inital information from the patient about why they were there. Then if the resident decided to admit to the hospital, we wrote the H&P. THis was interrupted regularly by calls to go deliver a baby (most students delivered a total of 10-15 babies during the month on OB. The gyn month had "call" but almost always the resident said there was nothing going on, go home.
Surgery was every 4th nite on general surgery. THis was different from the other rotations as only one student and one resident stayed. The residents were pretty good about only calling the student for interesting stuff. Trauma call was every other night...24 hours on, 24 hours off. The whole team was usually up most of the night. If it wasn't too busy, you could rotate sleeping for an hour or two. But we have a pretty busy trauma center.
And finally, family practice had "call" where you had to stay 2 days til 11pm. Often it wasn't busy, so we got to go home early.
Having detailed all my call for you, let me now say that things are very different this year. The reason for that is becuase the ACGME (a committee that puts out regulations for residency programs to follow) has now implemented regulations limiting the work hours for residents to no more than 80 hours a week. And you can't work for more than 24 hours, with no more than 6 hours after to allow you to turn over stuff to the oncoming call person (or to spend in lecture, etc). Also, there must be on day off every 7 and one full weekend off per month. THese rules take effect July 1, 2003. So most progams are experimenting with changes in the call schedule this year so that the are sure to be in compliance by the deadline (huge fines for violators). Many people in medicine think this is a bad idea, but the rules aren't going away. Some programs solve the problem by using a night float (people that are in the hospital from say 6pm-6am every day for a month, then you would rotate off night float) making it more like shift work. Other places are trying to avoid night float, and adjusting the call schedules so that overnight call occurs less frequently (no more call every other night!)
So, the world of medicine will be very different by the time you start. You should still expect to have to be up all night sometimes, at least as a resident. But that might be in the context of simply working night shift for a month at a time.
Rest assured, whatever call schdule you wind up with, you will quickly get used to it. It's not so hard to stay up when you are busy. YOu get used to sleeping for short times, in various call rooms, with various types and amounts of nosie around. You may even get to the point where you feel great when you've just gotten 2hours of sleep and function quite well the following day (until you get home, then it hits you)
I think you are being very wise to consider all aspects of the career you are considering. I'd also follow the advice of a previous poster and start working now to refine your study habits. I was one of those crammers. Cramming got me through high school and undergrad. It doesn't work in med school. Learning to avoid it now will save you lots of pain in med school
