What should I do about my mildly dementia mother who has slowed down eating, expresses tiredness, and sleeps way too long?

Well, if it were my mother and this were a sudden thing, I'd have her checked out in case of a silent cardiac or cerebral event. Have you reported any recent changes to her PCP?

Your profile says Moms 53 but think thats probably you. So Mom is in her 70s/80s?

You do not mention any Dementia being involved. I would take Mom in for a physical. Having Labs done regularly is important. A problem is solved before it becomes worse. Mom's thyroid could be off. She may need B12. Potassium is low especially if on diuretics. If she does suffer from Dementia, yes the Winter months can effect her. I have always had problems with the Sun setting early. It goes down, so do I.

Someone suggested always have bright lighting. The curtains opened to let that Sunshine in. Get her out in the Sun and fresh air. As we age, though, we aren't as active so we don't need that much food. Make notes on what Mom eats daily. I just read that a woman over 70 should have 1600 a day. I am 73 and I start gaining weight after 1500. (I have counted calories to keep my weight down since high school) You maybe surprised Mom eats more than u think. Maybe give her a small breakfast then a snack in between breakfast and lunch. Some thing small at lunch, like a half a sandwich instead of a whole one, a nutritional snack mid afternoon and then for dinner a small helping of everything served. Maybe a snack later. I cannot eat alot at any one time or I am very uncomfortable.

Perhaps it's time to bring hospice on board, as she may be further along in her dementia process than you realize.
You can call them and ask them to come out and do an evaluation to see if she qualifies.
Most people wait too long before they bring in hospice, when you/she could certainly benefit from them now.
They will have a nurse to come out once a week to start and aides to come bathe her at least twice a week along with supplying all needed equipment, supplies, and medications all covered 100% under your mothers Medicare.
My late husband who was under hospice care for the last 22 months of his life, slept anywhere from 16-20 hours a day. That can often be a sign of a loved one nearing the end of their life along with the not eating as much. But let hospice determine all that for you.

My mom has 'slowed down' in the last few years but is also 91. Everyone ages differently. I feel older than her sometimes.

Let her sleep. Her body decides to sleep when necessary. Don’t push her to eat more; that can have bad results. You can’t wake, feed or entertain her out of dementia. This is how she is now, so appreciate and enjoy what you can. Best of luck on this difficult journey.

Remember, the brain uses the most energy of the entire human body. As a dementia ptnt, your Mom is doing the best that she can. If she is tired, let her sleep. It isn't up to someone else to decide what is "way too long" to sleep when a person is battling a terminal disease.

I've taken care of my Mom (and my late father) for 10 years. I understand the worry when the longer sleep begins. My Mom built up to her current record over years. I started noticing about 5 years ago. I mentioned it to the doctor but it was a normal aspect to her disease.
My Mom currently sleeps 20 or more hours a day. She barely eats. And she's been on hospice for 3 months

It could be that your mother has more moderate than 'mild' dementia going on which is now showing up with a bad appetite and sleeping more and more, meaning she's succumbing to the disease process. Many people's loved ones with very advanced dementia feel certain they're suffering from 'mild cognitive impairment' when the reality is something entirely different. You can't 'do' anything about an exhausted elder who wants to sleep and eat less except allow her to do just THAT. Some things just cannot be 'fixed'. If you feel she's depressed, speak to your mom's PCP about it. Otherwise, just let her be and accept that she's tired and wanting to sleep more and eat less.

Best of luck.