How do you know for sure when your parent is suffering from either dementia/Alzheimer's and some Sundowners?

If you don't know how long your mothers been up sitting in her chair with the lights on, how do you know that she's getting up at 3:00 or 4:00 in the morning?
Perhaps your mother just has some insomnia, and instead of just lying in her bed trying to sleep she gets up to sit in her chair.
Your mother is young to have any of the dementias,(though there is early onset Alzheimer's)and really what you're describing doesn't at all sound like any of them.
And sun downing happens typically later in the afternoon going into the evening when yes, the sun is going down, thus the name. With sundowners folks are often much more confused, anxious, and aggressive. They can also pace or wander more then as well.
So what you've described doesn't sound like dementia or sundowners at all, but if you're concerned why don't you make an appointment for your mother to have a thorough check up/physical at her doctors. Any concerns and questions you may have can be addressed then.

Why do you and your mother live together at this point?
Does mother have any history at all of mental illness? Depression?
How does your mother go about her daily life? What activities does she have in home or outside of the home?

What discussions have you had with mother when, for instance, you find her sitting up at 3 a.m. with all the lights on?
What does she say when you sit down and talk to her about this?

Does mother have an MD?
Are you medical POA or on her advance directives?

Is there forgetfulness?

I think that you know already that your next steps need to be to the MD office for an assessment for Mom. Has this been discussed with her?

I look forward to your answers and hope you will participate on this thread.

Why does your mother need a caregiver at such a young age?

When was her last physical and complete blood tests?

Has she been screened for depression?

The first thing I would suggest is to make sure she has a DPOA (or springing POA) in place and you are granted total access to her medical records. Then do the things already suggested, take her to her primary and give them your concerns prior, then on to the testing. A neuropsyc exam is what I would do if other things have been ruled out and it can take time to set that up so mention it to the doctor while everything else is being set up you can always cancel it if another issue is found. I think it’s worth getting a neuropsyc exam anyway so she has a baseline for the future.

Most of all if you are feeling like there is something wrong there probably is so be proactive.

When you ask her why she's up sitting there with all the lights on, what does she answer? If you haven't asked, you need to at least start there.

At 67, this would be no ordinary cognitive "age related" cognitive decline. It may be ALZ, which is one type of dementia that can come on at a really early age. The only sure way to diagnose most dementias is to discount any other health issue with symptoms that can mimic dementias.

AnnReid mentioned thyroid and this is correct. Also, a UTI, stroke, tumor, vitamin deficiency (if she is an alcoholic Vitamin D is doesn't get readily absorbed), and other mostly treatable problems. She will need a neuropsych exam by a neurologist (scans, other testing).

BarbBrookly mentioned depression, and she should definitely be checked for this.

My cousin was 68 when they first saw obvious signs of a cognitive problem but it was masked by a thyroid issue: her ALZ made her forgetful so she wasn't taking her thyroid meds properly. This caused her to lose weight (she was already a thin person but became emaciated), which caused her prescription to not be appropriate since it is weight-based. It became a circular problem since no one suspected that she had ALZ... she was "too young". Now she is 71 and mostly bedridden. It's been a fast trajectory for her (this is not the case for everyone), so no time to lose with your Mom. I wish you all the best.

Llamalover47, An MRI can only rule out other issues, not diagnose dementia or alzheimers. I agree with Donttestme in that testing by a neuropsychologist is the best way to discern her issues. If you don't have one close by, it may be well worth the trip for the testing.

Even if she does not have Parkinson's this presentation can give you an idea of what neuropsycologist testing should be like. When my husband was tested, we were told he did not show signs of Alzheimers at that time. There is a variety of dementia diagnoses and it is very helpful to know which one to receive proper treatment to slow the progression.

If you are seeing her current lifestyle as a CHANGE from previous behaviors, it may indicate the beginning of declining cognitive functioning, but if her sleep patterns are typical of what she’s used to and making choices that she’s comfortable with, she may also be OK.

My mother developed different sleep/awake habits in her 50s, when my father was on a Nightshift schedule at work, and never returned to more typical sleep times until she entered residential care at 90.

I’d want to know scores from a thorough Thyroid screening, and that would show up in a comprehensive blood test too.

I am posting to bring up another set of options. You have an option not to get a diagnosis, to let go of "odd behavior" being anything that wasn't done in the past or is done by "normal" people.
My mother refuses to be diagnosed. We have the funds to provide 24/7 care in my parents home with my father who can stand, but not transfer on his own. We had 15 hr care before my mother fell (probably a TIA given the recovery) and her memory and behavior completely changed. She refused medical care so we called Hospice and because we had already moved to 24/7 care, they visited once a week and provided items (i.e. had we not had caregivers, they would have managed some other supports.) Now my mom is "back" ... sort of and using her walker all the time. She is still trying (stubborn and proud) to find moments not to use the walker. Hospice may discharge her today but her blood pressure runs about 200. I know, complete danger and could be regulated with meds (tested out on an urgent care visit she insisted upon), but she refuses meds (I don't take medicine! and shoves them away). We're all past fighting with her though we find moments to make changes...like the bedroom - set up just like the old one though smaller. She still tries to use her original room sometimes (no surprise).
She won't use the care bell (strung on her walker because she forgets it, will lay on it and got bruising on her ribs) so we moved her to a room next to the caregiver (we provide a room in the home) rather than the "master" on the other side of the house. My mother is up and to the bathroom 2-5 times a night and often wanders. Like a mother with a new born, we listen for movement and let her do her thing as long as she is not in danger. We'll (care team members) ask if she wants water.
Yes, something is going on and its her life. My mom is belligerent at times and sweet at times are we are never sure nap to nap who we will get. And...she'll not get a diagnosis from the only doc that would possibly do it...who simply says "she's getting old".
Life just is sometimes. I don't live in the same state. Visit two weeks every month to take care off finances and the house itself (caregivers can do that). This trip it was pool timer and door bell chime and cockroach elimination. It's crazy and I'm glad I have a technical brain.
May each make decisions and hold to them until another viable solution (idea relative to resources and practical to personal circumstances) comes along. Only guarantees are taxes and change.

If you suspect dementia, have er evaluated by either a neuropsychologist or neurologist, or generiactric specialist. They can do testing. Once you have a diagnosis, you go from there.

First start planning…who can be POA…see an elder care attorney to help with legal stuff. Also start planning for care…is there family who can care for her or will she need a facility? Who is paying for care? Does she have funds? Will family help pay or will she need to go on Medicaid? Medicaid rules vary by state, so find either elder care attorney or social worker can explain rules. Basically to qualify for Medicaid, she cannot have much in asssets or they will require her to pay. Also, she can’t give away her assets…Medicaid has a 5 r look back at financial records…any funds magically disappearing in prior 5 yrs of her applying for Medicaid will be examined.

Care in a facility or having home care can be very expensive. Having family do care can be grueling…it’s a very hard job.

Get diagnosis first, then attorney and or social worker to help with plans and rules.

Your mother should be assessed by her PCP who may refer her to a Geriatric Care Specialist; between these two professionals assessments can be performed to further identify your mother's cognitive status. Your should also provide input to the PCP about your observed behaviors. Definitely get her to PCP asap to get started on the assessments for both your well being.