Hi there maryjoan, as the illness /syndrome develops the person suffering dementia goes through so many changes. Each change is individual to your other half. But do not forget that you also go through these changes with him. Try to think about this and it might allow you to get an understanding of the process as it continues.
As I see it you are seeing changes to his behaviour. You say he sits in front of the TV all day, does not move and sits up late. There is your answer- as he goes through the stages of dementia and because of the Tangles associated with the dementia in the brain messages in the thinking part of the brain get mixed up and that is why he is doing what he does. To him he might think that he is at work and doing a really good job even though he is just sitting there doing nothing. Here comes the tricky part because he is doing nothing all day just looking at the TV, he is not using energy and it gets stored up so he does not need the same sleep as we do. Possibly 2-3 hours and he is again very refreshed wanting to start his day as the day before and the day before that, and the next day to.
Do you ever try to make basic conversation with him or as it might seem silly to you play some mental games with him so that his brain has been stimulated rather than watching Jeremy Kyle and all the other day time rubbish that they put on. If his brain has been stimulated then the energy is being used so he will sleep for longer periods. Some form of exercise will help get the surplus energy removed from his body. Going to day care 3 times a week can also allow you to have the time to yourself. Then there is respite care at a local nursing home for a week or so.
Contact your social worker or care of the older persons mental health team for assistance. Don't live with this problem alone because with out proper sleep yourself you are going to become more stressed than you are already now. Best regards Oldman1952