I saw an article about the nuns, but I guess the person that was writing it was using it as an example of meditating. They were saying the nuns they studied had practiced a certain type of meditation daily.
I bet not having children is probably part of the reason, but I wouldn't rule out meditation as part of the reason. Just from my experience with anxiety I realize how powerful "mind over matter" is. I've given myself real symptoms of diseases I don't have.
Plus, the clergy are supposed to be the happiest people when grouped by careers. This is coming from the Oprah Show, but I find that a reliable source!