IMHO it depends on the timing of your leave of absence and the structure of your program.
I cannot fathom that a request to leave mid-year would be viewed favorably, or even charitably. And I wouldn't only worry about my PD's reaction but rather, my colleagues...after all, presuming you are in a specialty which takes call and has clinical responsibilities, if you leave, your colleagues will have to pick up and the slack (ie, extra call, extra work, etc.). And as much as they like you and would support your goals, when you're tired and overworked, its hard to be understanding.
If your program has lab years, or frequently has people taking a year or more off for other projects, then a leave of absence might be more acceptable, as they have handled it before and may have provisions for such a case.
Regardless of when you leave, as much advance notice as possible would be appreciated, as I assume your PD would try and fill your spot. The later you notify them and the later in the year you want to leave, the more difficult it will be for them to do so.