Is a 16 hours + sleep night normal with dementia?

Hypersomnolence is common with dementia. It is even one of the notable characteristics of decline, and a risk factor for progressive loss of function. That said, it is also a characteristic of depression and some endocrine disorders. As others have said, discuss with the doctor.

Charly24: After checking with the person's physician for underlying causes (think U.T.I.), perhaps they do require sixteen plus hours of sleep.

I am an advocate for letting our elders live their choices (food, sleep, communication, etc) as long as they are clean and safe. We can't fix old, we can't fix dementia and we certainly can't fix personality disorders (without medications)as they say, "to each his own."

We have a very old cat who sleeps endlessly, sometimes we wake her up just to let her know that she's still alive. LOL

I think that's really common and although you should make sure there's no underlying illness (UTI, for example).

My MIL with (undiagnosed) dementia sleeps between 18-20 hrs per day, most days. 14+ hrs in bed and then sleeping off and on through the day in her recliner.

Honestly, it makes it a lot easier to deal with her.

Yes

My mom sleeps 16 hours and then naps between her meals. She has vascular dementia (dx 2020) and has been doing this since the beginning with more awake time during the day but as of 2021 has maintained her sleep patterns. The doc told her that her veins don’t stretch which means she experiences pain like a heart issue. She was monitored and treated for depression but that didn’t work for her. She is happy with her bed. I have 7 day a week part time caregivers who make sure she eats 2 meals a day and gets social cueing, showers, lotions, warm compresses for her eyes, hair done and nails painted along with getting her feet rubbed. Bubble of bliss for mom is sleeping!

Age and declining condition lead to longer sleeping more than dementia itself.

We often discuss her the enormous amount of sleep elders begin to want with advanced age and/or dementia.
You should discuss with doctor ways to get your elder up, repositioned, fed and medicated, but do also understand that sleep is their preference often enough. While a bit of exercise and being up in chair goes a long way to keep the bones healthy and the skin from breaking down, often this is a losing battle in advanced dementia.

Wishing you the best.

As a person declines they will sleep more.
My husband went from sleeping a "normal" 8 hours to 10 then to 16 and eventually 22-23 hours a day. Increased sleeping is but one of the declines Hospice monitors for recertification

Your wife according to your profile is only 67 years young, which probably means that she's been diagnosed with early onset Alzheimer's. And that as you know can go on for 20+ years, so the fact that she's sleeping this early in her diagnosis is to me not normal.
Sleeping that much and more is normal for someone further along in their dementia journey, so I would talk to her doctor, as your wife may be suffering from depression which too is common with dementia sufferers. Or perhaps if she's on any new medications, that could be the culprit as well.

My late husband who had the most aggressive dementia of all, vascular, slept 16-20 hours a day for about the last 6-8 months of his life.

I wish you the very best as you take this very difficult journey with your wife.

I found that to be the case with my dad.