I don't think a person can keep up a charade indefinitely. Eventually you either get caught pretending or you get tired of faking it.
I have found simply stating that you are "taking a break" from the routine works well in many cases. But it is good to avoid saying too much as to why.
I just say I felt I needed to take one and that I feel quite good getting away from all the politics and stresses. You might have to take a few lectures but they often find themselves in a difficult spot trying to get you to go back to the misery they so often hate as well.