Have you investigated sleep apnea? My husband also sleeps until 10 or 11, but I think that can be a dementia characteristic. He doesn't get sleepy or nap during the day, but at 92 he is fatigued. He has used a CPAP since the early 2000s and I see a direct correlation between the his apnea events (stopping breathing) and HOW fatigued he is during the day.