Why is he sleeping so much?

My husband was recently diagnosed with ADHD. He got prescribed Dex. He takes it when he wakes up. He only has a part-time job, so he has to go to work only twice a week. On work days he gets up early with an alarm. Then at night he either stays up late or takes a nap for a couple of hours in the evening and then wakes up to stay up until sunrise. When he doesn't have to get up for work he doesn't set the alarm and sleeps in till 4pm. Then takes a nap in the evening. Sometimes he wakes up for "breakfast" which is already lunch and goes back to bed after he eats. He wakes up when it's "normal bedtime" and again stays awake at night. Sometimes he goes to bed early at night, sleeps till 6am, then goes back to bed at 10am and sleeps through the day. He takes the meds when he gets up, so if he doesn't set the alarm he just chooses to sleep in because he wants to. Then he gets frustrated that he hasn't done anything. His sleeping looks really irregular and sometimes adds up to 20 hours a day! His psychiatrist checked for depression and decided that there is no depression. He just gets upset with himself because he can't get things done because of ADHD. I ask him why he stays up so late and doesn't just make himself go to bed: he doesn't know. When I try to talk in the bedroom he has no problems falling asleep the minute his head hits the pillow.

Is this pattern common for ADHD? OR is it depression? Or something else?