Muscles are muscles, and you drain them by using them too- it's just that they come back stronger.

I think willpower is actually like this, but my only data for strengthening willpower is personal/anecdotal.

This seems to be similar to how people behave morally- there's the consistency effect that reinforces itself ("I am someone who gives to charity (clearly, since I gave to charity last week) so I'll give again"), but there's also the feeling that you've given enough so you become less charitable shortly afterwards (wasn't there a study that found that people coming straight from church tipped less?)