I have intermittent insomnia myself so you have my sympathies. But yeah, this would make teaching potentially rough--I mean, again, you can pick your days, but you also can't know in advance what days they'll call you. Also, I don't know about you, but I don't do well when my sleep (when I can get it) is interrupted, and getting morning phone calls on the days that I do need morning sleep is a pretty good way to get my sleep interrupted. (Agh, I have no idea if I'm phrasing this well.) Tutoring, besides being more flexible, also has the advantage of generally taking place in the afternoons or possibly other times during the weekends. (I've only tutored through college centers, not on my own, but the logistics are pretty inarguable.)