Will Sundowner's shorten your life span?

Sundowner's is the confusion that people get in the evenings. Nobody knows for sure what causes it. It has not been linked to shorter lifespan in any studies that I know of.
However, there are tips to help deal with it.
Turn on more lights in the later afternoon/early evenings so lengthening shadows and darkness to not create confusion.
Keep a routine lifestyle and consistent environment. This helps people know what to expect in their environment and their day-to-day life.
If the person experiences a lot of agitation or threatens violence, consult his/her doctor. The person is experiencing high anxiety and their fear is making them want to eliminate a perceived threat. Medications can help calm this anxiety.

If your loved one is getting 7-9 hours of rest throughout the day and evening, he/she is getting enough rest. You might want to limit daytime naps to help him/her sleep more throughout the night. If that doesn't help, talk to doctor about medications to help induce sleep or maintain sleep.

CoolBaptist: Restorative sleep is a necessity. He should see his physician or neurologist.

There is a condition called terminal insomnia. Maybe google that

I don't think it has a direct correlation to shortened life, since dementia may already do that. My Aunt had dementia that we began to recognize when she was in her late 80s. She lived to be 100 (until she fell and broke her hip). She sundowned every day. FYI I'm not sure you completely understand what sundowning is:

"The term "sundowning" refers to a state of confusion occurring in the late afternoon and lasting into the night. Sundowning can cause different behaviors, such as confusion, anxiety, aggression or ignoring directions. Sundowning can also lead to pacing or wandering."

Source: https://www.mayoclinic.org/diseases-conditions/alzheimers-disease/expert-answers/sundowning/faq-20058511#:~:text=Answer%20From%20Jonathan%20Graff%2DRadford,lead%20to%20pacing%20or%20wandering.

I suggest reading the whole article since it gives all of the behaviors and strategies to minimize it.

Sundowner's might shorten the caregiver's lifespan. :-)

I would say no. It is the dementia that will take him down.

He will not sleep as you do, with dementia everything changes.

It's probably not the "sundowners" that will shorten his life but the dementia itself.
Like already said there are medications available for those suffering with sundowners, so make sure you're talking to his doctor about it.
And know that there are no happy endings with dementia only death.

What indication(s) do you have that demonstrate to you that “…this really affects his mind”?

In mid-level dementia, especially with medical conditions such as your dad’s, his sleep levels mayor may not have any relationship to cognitive functioning.

See his MD regarding sleep pattern. Medications can help.

As to question of life span, no obit was written for someone in 80s and 90s where people conjecture about how shortened their lives are; that's a good long life well past life expectancy stats. No coroner's exam ever says someone died of lack of sleep, so any such guess would come under a general "failure to thrive". However, sleep makes a difference to our mental status, and it is best to get a good sleep pattern going if this is possible. Sure do wish you good luck. Work with the MD.