I have a deviated septum which makes it almost impossible to breathe through my nose, and was made worse by an operation supposed to correct it a while ago, to the point where there are days where at least one nostril literally can't move any air. So I'd, like, get rid of the whole damned septum and replace it with one that does what it's supposed to.
It's that, or my teeth. The buggers constantly hurt despite, according to several dentists, being perfectly healthy. But then I wouldn't be able to chew.
If we're talking any old biological process... sleep and fatigue, I guess, since I'd be soooo darn productive without it.