Instead of lying down or staying relatively immobile all day and then sleep for 12 hours. She won't try to move at all or attempt to engage her mind in anything that requires thinking despite her physician stating these activities would help her greatly.
What were your mother's activity levels before the dementia? What kinds of things did she do during the day?
Is she depressed? Is she being treated for it? Have you discussed her excessive daytime sleepiness with her doctor?
The doctor is right that physical activity is important in the management of dementia symptoms. Social interaction is also important. It is worth some effort to figure out ways to improve these for our loved ones.
Give us more information, to make our answers more appropriate to your situation.
What did she enjoy doing before she developed dementia?