The theory of 3 point restraints (i.e., lap and shoulder belts) is that they give much more protection to your upper body by restraining you from flying forward into the dashboard in a collision. That's why automakers and regulators switched from the original lap belt design to the lap and shoulder belt.

But when you are lying down, you don't want the shoulder belt on you. All you want is the lap belt to hold you into your seat. Then, if you fly forward, you won't hit the dash, and you also won't ride up on the shoulder belt.

So when you recline back, put the lap belt around your waist but then put the shoulder belt UNDER you. The tensioner should still stop the belt from loosening in an accident, if it is working properly.

This isn't as safe as not sleeping in the car at all, but remember, sleeping in the passenger seat is often what makes long drives possible with 2 drivers.