Clint Faraday Mysteries Book 40: Dead Tired Collector's Edition

A man is dead tired all the time. He gets plenty of sleep, but has strange dreams and wakes up exhausted, sometimes with cuts and bruises. They decide he is a sleepwalker. He can stop it by taking medication. Then he dies in an accident on his ATV. While asleep. There is something else behind it. What does radioactive gas have to do with it?