My husband says that sometimes when he's falling asleep he'll hear a loud surge in his head with flash of bright light. only occurs once. ehs?

"Exploding Head Synd: What you are describing sounds like the dramatic sounding condition known as "exploding head syndrome.". It is a type of sleep disorder known as a "parasomnia". In spite of its frightening name, it doesn't have serious health consequences, The only treatment I am aware of is clomipramine,, an antidepressaant, but it eventually resolves on its own. Consult with a sleep specialist may help.