Well, I would say that "thinking like a computer", as you mentioned, is not necessarily a bad thing, and is actually quite far from being a sign that you need a break...

I was even taught that this kind of "meta language" should be the first step a programmer should do before starting any big task/project... But hey, we know that's almost never possible anyway...

I have recently been inflicted with a nasty headache, coupled with stinging pain in my eyes, whenever I spend too much time in front of either a computer CRT or an LCD. And unfortunately, I don't even have to be programming, so even if I just kick back and browse, it'll come...

So, to sum it up: don't wait until you discover your "break point". Just go ahead and take a break/vacation/leave whenever you can, don't wait for it to become an absolute necessity.