I only ever have one type of nightmare - the zombie apocalypse nightmare. The locale changes, but it's always the 'Night of the Living Dead' scenario where I'm trying to keep them out with a hairbrush and a flimsy screen door. And running from them through conveniently-placed quicksand. You know the drill.
I've recently come to the conclusion that they represent the fear of being caught unprepared. It's not my only fear - spiders, ew - but it certainly encompasses a lot of them. Because you can prepare for floods, hurricanes, losing your job, death and dismemberment, but who the fuck prepares for zombies?? Being prepared is kind of how I function (yeah, that's right, I'm Batman). I can't leave the house without running through a bunch of different scenarios in my head, can't pack a bag without making sure every contingency is covered.
(Although, this is in complete opposition to the way I parent. I'm a study in contrasts).