The Oatmeal has a comic out that aptly describes how your brain torments you as you're about to fall asleep.
My brain is a vengeful thing, I think. I get awesome ideas when I'm driving. But as I tell Ranger, there are only three things I can do when I am driving: drive, sing, and talk.
I get great ideas for texts and twitter posts just as I am stepping into a crosswalk. And sometimes I am minding my own business when suddenly I MUST LOOK AT MY SOCIAL NETWORK. Almost as if a button is pressed when the walk sign lights up.
And of course my brain is a flat out creative genius when my hair is full of shampoo.
The reason I say my brain is vengeful is of course the whole sleep deprivation thing. Possibly the caffeine thing, too. Thing is, when I do have time to sit down and write/draw/craft/create, that is when my brain is most petty. Because when I have the time, I can't remember what those awesome ideas were.
Where do your ideas ambush you? Cooking dinner? Bathing kids? Biking?