No, you are not required by federal law to provide a shift differential to exempt employees. Many companies would, however, if they provided one to non-exempt employees (especially if they worked together on the off shift) and to recognize the negative effects shift work has on employees and their families. There is no legal requirement that I know of to pay a shift differential to non-exempt employees.
Call a few companies in your industry to see how they would handle it. Some companies pay different shift premiums for the two off shifts. I belive the midnight shift is more difficult than the afternoon shift, in terms of impacts on your health, getting used to it, and having normal familiy and social life.
Here's a whole website devoted to shift work: http://shift-work.com/shift-schedule-issues/shift-schedules/
Search on shift premiums here for previous discussions.