I don't know if you specifically have my trouble with being awake in the morning hours, but given what grade levels you'd be substitute teaching, that could be an issue too.

The insomnia is a major reason I try not to schedule mornings in the course of my ordinary life. I mean, on Monday I got up at five-thirty in the morning after two hours of sleep and ate breakfast and packed and worked a little and caught a train at eight-fifteen—and then I passed out awkwardly for the next four hours. That's not an option if I have to be somewhere at eight-fifteen and then mentally functional for the next eight hours. It's one of the things about a steady job that worries me a lot.