Hi, here is an part of an article I read which has a fuler answer to your question.

It is quite common for a person with dementia, especially in the later stages, to spend a lot of their time sleeping – both during the day and night. This can sometimes be distressing for the person’s family and friends, as they may worry that something is wrong.

Sleeping more and more is a common feature of later-stage dementia. As the disease progresses, the damage to a person’s brain becomes more extensive and they gradually become weaker and frailer over time.

As a result, a person with dementia may find it quite exhausting to do relatively simple tasks like communicating, eating or trying to understand what is going on around them. This can make the person sleep more during the day as their symptoms become more severe. 😊🌹

Because their brain is damaged by the disease.

In the earlier stages, I figured that it must be exhausting for them to constantly be trying to figure things out in their mind. Kind of like working at a computer all day. It leaves you feeling drained. Depression will also cause them to sleep a lot.
Just my humble opinion. 🙂