Duke currently has a hybrid system for interns:
1) Q4 overnight call X 3 months (CCU, Gen Med VA, Gen Med Durham Reg)
2) Q5 overnight call X 2 months (Gen Med Duke)
3) Night float X 3 months (Oncology wards, Neurology wards, Cardiology wards --> typically 3 weeks of days and 1 week of nights per month)
4) No intern call or night float X 2 ward months (MICU, Renal, Pulmonary)
5) No intern call or night float X 3 months (Ambulatory, Elective)
This adds up to 13 months because the schedule varies a little bit for everybody in terms of specific number of weeks on each service (e.g. -ICU blocks are 3 weeks, some rotations are 2-3 weeks long, etc)
I believe this is a nice way to experience the pros and cons of both systems.