When supine, he had severely reduced blood flow through his brain returning to his heart.

Reading is a quiet activity and it will often induce sleep in the supine position. Especially if it's a dreary textbook or work paper.

In fact, healthy people never sleep on their back.

It is a medical observation that sleeping on the back is the distinctive feature of sick people.

It is only when body oxygenation drops, breathing becomes heavy, and symptoms of various diseases appear, sleeping on the back becomes normal.