I am curious, should I let them sleep as much as they want?

Yes, I would. Some disagree. But if she's getting up enough to keep her skin conditioned, and get a bit of circulation going I would allow her to sleep all she wants.

Sure, why not? At this stage she most likely doesn’t need more stimulation, and everyone is better off not having to entertain someone who doesn’t respond well anyway. Her body knows what it needs, so let her be.

Absolutely you should. It sounds like she's quickly approaching end of life and needs a hospice evaluation stat.

My Husband went from a "normal" 8 hours of sleep and as he declined he would sleep 10 hours, that became 12 hours and the last few months he was sleeping 20 hours and the last few weeks he slept 22 to 24 hours. I would wake him for a "meal" and to change him. Sometimes he would sleep through a change or would sleep through a sponge bath.
Sleeping more, longer is one of the declines that Hospice will take into consideration when recertifying someone for Hospice.

I think because their body is failing, they need that sleep. I let my husband, who is at stage 6, sleep whenever he wants.

Her body is shutting down. I would let her sleep if she wants to sleep...

My husband seems to be mid-point in his Frontal Temporal dementia. Over the past 3+ years he has gone from waking up around 11 am to about 1:45pm. He may be going to bed later-I'm too tired to stay up very late to find out.
Sleeping is totally fine. This is what his body needs, that is ok.

Let her sleep. What is the worst that could happen? Most people pray to pass peacefully in their sleep. Physically her body can no longer produce the energy to keep her vibrant. I have been a hospice nurse for years. The greatest benefit hospice would have for your sweet mother is that they provide her with equipment such as bed, bed trays with wheels and supplies like pads, diapers etc. Also any medication for comfort if she needed it. She may or may not need hospice. That’s a family decision. I can tell you some patients who are still sharp and alert do not like the stigma of hospice. You know your mom best. Sounds like you are doing a beautiful job. Don’t second guess yourself and relax. Use the quiet time doing something for yourself.

She's shutting down. It is time to call hospice for an evaluation. Let her sleep.

Agree. Call hospice to come and assess her and review their services with you and family.