In truthfulness to yourself and the potential future employer, you should be as honest as possible regarding what your availability is when it comes to nights, weekends, and holidays. While reiterating that you are open and flexible, make sure that your interviewer knows of any personal commitments that you have because it is important that these things are discussed during the interview process. Based on your research into the hours requirements for the position, make sure to reiterate that you can handle potential 12-hour shifts and even required overtime. If needed, you can always ask clarification questions to your interviewer.