What Causes Low Heart Rate With Excessive Sleep And Fatigue?

My father age 69 has worked hard labor all his life. 2 years ago he was suddenly burdened with migrane headaches, today he has heart rate as low as 40through out the day and O2 in the mid 60 s when sleeping. Doctors say these are ok and nothing to worry about. over the last 2 years he has napped longer and longer.. the past 6 months he is sleeping 22 hours a day only up to eat. He feels as if he has the world by the tail when he wakes in the morning but by the time he does his morning routine, gets dressed his energy is zero and he can not walk to the bed alone. he is on a bpap at night 12-14 with o2 at 1.5 ideas on whats going on??