My experience in Somerville is years out of date, but I would recommend strongly against substitute teaching if you have stamina issues. (I do now, which is one reason I'm not doing it myself.) I mean, yes, it's flexible in that you can choose which days, if they call you, you choose to go in. But once you show up, you're kind of stuck for the rest of the day. You might luck out and get a well-behaved class whose normal teacher is on top of it. Or even if the normal teacher is with it, the class might be having an unruly day. It's kind of a crapshoot.

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.