It usually strikes at the worst times. Early in the morning when I can't sleep, or late at night before falling asleep. When I wake up and try to go back to sleep. (Notice a pattern here?) Now, late night writing sessions aren't unusual. It's when I get most of my writing done, even into the wee hours of the morning when all the world's asleep and there is no one to disturb me. But it's those times when I just want to switch off, that I just casually start to think about a scene, or a character, etc. that ideas come flooding in. Dialogue starts to form. Actions play out. Introspection is realized. I try to tell myself that I'll remember it tomorrow, or when the time comes for me to actually write the scene. However, I know that's not the case. I will forget.
This is why I keep a notebook and a pen next to me in bed. My sheets are filled with ink stains. (Finally got rid of that pen.) I now have several notebooks filled with scenes and ideas and late night scribble that I can't decipher.
Maybe this is why I am tired all the time. It's the price I have to pay for this madness. But I will pay it gladly.