How do I re-regulate my sleep schedule?

Okay so right now my sleep schedule is pretty messed up. I don't go to bed until 9-10am and wake up anywhere from 4-6pm. The problem is, even if I get 8 hours of sleep I'm still totally exhausted because my body absolutely hates this pattern. No matter what I do though, My sleep schedule always goes back to staying up all night and sleeping all day. How do I fix this and keep my body from reverting?