Although there are both federal and state labor laws, this is an area where state law probably controls. You can contact Wage and Hour (or its equivalent in your state), but my shoot-from-hip guess is that it's legal. If it's legal, there are a couple of other issues to look at: Does your employer give new hires an Employee Handbook? Courts in most states have held that such a handbook constitutes a binding contract between the parties, in the absence of contrary statutes or state or federal regs (that's one reason that some employers don't have a handbook and most of the ones who do keep it vague). If you have a handbook at your workplace, it may say something about an employee's obligation to work overtime when needed . . . but I doubt it. If the handbook is silent on the issue, you, as the employee, can argue that you never explicitly or implicitly agreed to work overtime on demand . . . of course, the employer will argue that it can make reasonable demands of employees with respect to hours, etc. Also, is your institution covered by any sort of collective bargaining agreement? I know that Wisconsin is a heavily unionized state in many areas, but I know nothing about their health care system. If there is such an agreement, ask the shop steward what it has to say.
Finally, I doubt that an employer would simply fire you if you refused to pull a double. They probably would be too concerned about liability for wrongful termination; besides, so as long as you are past your probationary period, how could they prove that you didn't have some higher legitimate priority (although I don't know what could be a higher priority to you than having the ability to say how you are going to spend your time).
Short answer: Absent any of the above issues, it is probably legal for the employer to ask; it probably is also legal for you to refuse.