Since my mother (aged 90) developed vascular dementia nearly 8 years ago she has needed a lot of sleep. She has her breakfast in bed at 8am then sleeps again until about 11am. Her carers get her up and she goes to the living room where she often naps until lunchtime. By 2pm she is exhausted. She goes back to bed from 2pm until 5pm. Then she gets up and goes back to the living room, having her supper at 7pm. She goes to bed at 8pm and sleeps for nearly 12 hours at night.
I think that, like a baby, she often wakes and lies thinking, but has not got the physical stamina to be 'up', even sitting in her wheelchair, for longer than she currently manages. Her GP says that she has no physical reserves of energy. On days when she has a morning shower she is really tired.