Why does someone with dementia sleep so much?

My mother has gone through 2 brain surgeries back to back within 3yrs , she had two 6cm Atypical Grade 2 meningiomas removed on her frontal lobe.This last surgery she had to go through 28 radiation treatment in which she was doing pretty good after that. Then February of 2013 she fell and broke her right hip and has declined since then. She also has dementia and is on so many different medications. She sleeps nearly all the time, she only mumbles and can't speak or comprehend anything. Even when she is sitting up she is leaning over sleeping. She is being treated for underactive thyroid also.