I found a 30 degree bag to be more than adequate for Southeast Alaska, which I'm guesstimating gets about as cold during summer nights as Michigan. I used to sweat a lot in that bag. My favorite one for summers has been a bag I made out of a military poncho liner. It's kept me warm (when I'm almost fully clothed while sleeping inside a bivysack) down to a breezy 35F. I think a good compromise would be a bag that's rated down to about 40F.