Dementia senior more alert in sleep state than in waking state?

Hi there: My father (age 84) has middle stages of dementia (likely Alzheimer's). He finds it difficult to form words and it is painstaking for him to speak while searching for those words and he is constantly getting lost in the house, mistaking one room for another. Last night my mom forgot to give my dad his Melatonin so he could sleep through the night and needless to say he was talking up a blue streak and kept my mom up the whole night. The interesting phenomenon here is that in his "sleep" my dad could remember all the words. He was speaking about things from his past and choirs he had directed and so on. He had no issue at all with remembering and getting the words out. How come no one has ever tapped into this, e.g., finding out the correlation of a seeming subconscious mind in the clear and transferring/transmitting that state of being to a waking state so that the individual can articulate and remember and function in society? I think this could be truly amazing.