Mom with dementia seems to be sleeping more and more. What does this mean?

Cont...at all. This has resolved since Mom is now living in the memory unit of an assisted living. She is kept busy with all the activities offered and eats three healthy and delicious meals a day. She packs her things and says she thought I was coming to take her home when I visit, but I have to keep reminding myself that she's receiving better care than I could have ever given her alone.

My mother was staying in bed all day. She would get up for dinner and watch a little TV and go back to bed and sleep all night. I called the memory center where she was a patient and they told me people with dementia sometimes sleep alot ornot

Is she on a daily vitamin? Eating enough protein? Nutrition declines in many elderly (not just dementia), & key nutrients (b vitimins, etc) are absorbed poorly in elders. Meds can interfere and diet variety may be limited (less fruit & veggie). Caveat is that not all people absorb vit. pills well, so there may be some tinkering needed (ie I can take vitamins, but w/ "vitimin & mineral" suppliments my face breaks out -- just wierd biology thing I guess).
You didn't say if she had a caregiver, but maybe someone to swing by daily & check meds, cook a meal or "clean house" for an hour?

More sleeping could mean that she is overmedicated. It could mean that she has an acute illness, like the flu or a uti. And it could mean that the dementia has progressed and she is in the end stage. Perhaps it would be helpful to keep a journal for a few days of her sleep and any other symptoms and her confusion and discuss that with her doctor.

If/when she is in the final stage, I highly recommend having hospice care come in to the house.

Memory loss varies from one type of dementia to another, but in general it is short term memory that suffers the worst. My husband might not remember what he did yesterday but he could tell detailed stories about his school bus driver from 75 years ago. Remember details from long ago while having lapses about more current events is just the nature of the disease.

My mom has dementia and is 90 so she tires more easily and sleeps or cat naps most of the day away. She doesnt' have nor want much activity so I don't worry about it as long as she is happy. I do find that the more she sleeps, the groggier she is and the less engaged she is. When I'm with her, I try to do more activities and balance with rest or turning in early in the evenings. I know once I leave, she sleeps for a few days to catch up.

I wouldn't worry too much unless she is missing meals, losing weight, or lack of activity is robbing her of any lucidity.

Have you checked with her doctor? Might want to make sure her health is good, she isn't anemic and she is getting enough nutrition as well. Maybe find someone who can visit her a couple times a week, make a daily phone call to her or invite her to senior center to keep her mentally stimulated and having something to look forward to during each day.