I do know of at least one situation where it is possible to "break shifts".
At my medical school, one of the big name ED attendings would only work 4 hour shifts. He did this by finding someone else in the group who was willing to work the other half of his normal 8 hour shift. It probably helped that he was/is one of the bigger names in EM, and easily the biggest name in the department in terms of research productivity and national speaking / recognition.
It may be possible to work a situation like this in a private group if you had someone else who also wanted a similar schedule to you and was willing to split shifts (and salary) in order to attain this goal.