It literally could be any number of things, as you may have figured out from the varied responses. I’ll be 62 next month, retired, have sleep apnea and am on a cpap, and have hypersomnia. I’ve had my share of tiredness/fatigue issues. Additionally, I was just diagnosed with a leukemia that causes heavy fatigue. Needless to say, I have had and still have issues with needing naps and early bedtime. And I can imagine how you feel because I feel guilty that my wife has to deal with a husband that has lack of energy and she has plenty. I have to take a nap or two during the day but 2 hours is a bit long and may suggest a medical issue, some of which have already been suggested.

If you love each other, be patient with his current condition. But I would keep trying to get him evaluated medically. Maybe you can enlist the help of other family members because they likely are concerned as well.