Demand can be very bursty for nursing.
My wife works in postpartum care, and while she usually works 3 12.5 hr shifts per week, if the hospital is going through a busy time it can be more. In addition to her normal scheduled hours, she will have a few on-call shifts per month, and during feb-april (usually the busiest time of year around here) it's pretty much assumed that she'll get called in for these. In addition to the scheduled on-call shifts it's usually not hard to find other times that the hospital is in need.
Of course even at that, I don't think she's ever worked more than like, 60 hrs a week. I can't imagine anyone working 80 hr weeks consistently in nursing. It doesn't even seem safe - nursing is very physically and mentally demanding work, someone consistently overworked is much more likely to make mistakes.
