Lyn's pattern of sleep has changed. She's now in bed around 7pm instead of the 9 or 10 pm that was her norm up until a month or two ago. She's also sleeping later in the morning. If Mom has to wake her, she's surly and acts as if she's not had enough sleep.
Sleep changes are common with dementia and scientists have not yet figured out why. While she is sleeping more currently, this too will change. Most likely, she will start having interrupted night-time sleeping and more daytime sleeping.
For Sleep Changes - Alzheimer's Association
Sleep Problems in Dementia - Family Practice Notebook
Dementia - Behavior and Sleep Problems - Medline Plus