Thanks for bringing this to my attention! Sleeping in a tent or shelter indeed raises the temperature, but this doesn't seem to work well (unless during winter season). I will try to fix this with the next update
Red51, i'm afraid you forgot to repair this bug? I have still that issue in the last update (0.8.5. fishing ...) but now even worse ! Sleeping in the shelter get you overheated in a few minutes in all circumstances. I use the shelter mainly to sleep in the night so it will become instant light (all other players sleep also).