What could cause heightened heart rate and breathing in sleep?

My husband is always tired, even though he takes frequent naps. I've noticed that, while asleep, his heart races, as though he's just had a fright, and his breathing , likewise, is heightened to a frightening level. He is only 34, but has the energy levels of an old man. What could cause his heart to labour so? I know that this will impact his oxygen levels and therefore his energy, and could cause him to exhaust his heart.