How do you get them to go to bed and not be up til 3am?

My mom also has dementia and I had the same problem. Her doctor prescribed her something that makes her tired and sleep through the night. Ask her doctor , I'm sure they cane help. Now my mom goes to bed at 9pm and I get a couple of hours to wind down before bed.

Your profile says that your mother has dementia. Often people who have dementia have sleep disorders. It's rather common. I'd discuss it with her doctor to see if there is some medication that might aid her in sleep. There are a number of options. I'd hesitate to use anything without doctor's recommendation.

There are other reasons she could be getting up. Is she going to the bathroom and once awake has difficulty settling again? If so, eat an earlier dinner and consume fewer liquids before bed.

Is she not not getting enough exercise during a typical day? Try more physical activity. All of us sleep harder and better (and wake clearer) with a bit of physical activity.

You can also add more mental stimulation during the day. Try to limit daytime naps to thirty minutes or less.

There are medications that can help.
I would try to avoid the "PM" medications and Benedryl. They make my brain "foggy" in the morning. I realized after taking one one night and waking up foggy that Benedryl is not a great thing and giving it to my Husband that had dementia might be making his poor foggy brain even foggier.

Melatonin can help and it is safe BUT once a person is asleep if they get up and turn on a light the light disturbs/destroys the serotonin that the melatonin helped. So if she needs to get up to go to the bathroom often it might not be a good solution.

I would talk to the doctor to see what safe effective medications might help. Keep in mind many have "sleepwalking" as a side effect so someone will still have to be aware if she gets out of bed. At that point maybe a bed alarm might be an answer.

Has this been happening for along time? If not it might just pass and be replaced with another behavioral pattern. Over the years my wife has ran thru a bunch. They always pass. Always. 

My SW did a similar thing. I kept a log and found she got up 70% of nights and went back to bed 60% of those nights. So out of 100 nights she got up 70 times and back to bed 42 times. Overall she got enough sleep. There was no pattern; it was just random behavior governed by a demented mind which works at random. No docs. No meds. I toughed it out and it passed. A few months. Was it easy? No!

The log was an attempt to pin down a possible outside influence which might be causing the above behavior. Nothing ever presented as cause and effect. This led to a re-analysis of the records thence to the conclusion of merely random behavior.

Is she sleeping at some other time, or just not needing sleep? If she sleeps at other times then you have two choice, to stop her doing so in the hope she then sleeps at night or to let her and accept she is nocturnal which doesn't fit with other's lives very well. I wouldn't bother with Benadryl, plus you don't say what else she is on. Speak to her Dr and get some prescription sleeping tablets if you think these would help, or break a sleep pattern habit. Is she a danger walking round the house? Or does it actually not matter if she is up when you are asleep?

Speak to her doctor about altering medication to help with this. Mom had same issue, would get up if she didn’t fall asleep right away (like within 10 min which was as soon as I was in bed) she’d be up with lights and tv on. Then would go back to bed, up again etc. Melatonin can help but for her had the opposite effect. Mirtazipine did the job.. It’s an antidepressant that helps with sleep. She gets it about 1/2 to an hour near bedtime and then she wants to go to bed. I also turn off lights if we’re watching tv to help set the mood for nighttime/sleep. Pull shades, have low watt bedside lamp. Put my pajamas on too- all to signal nighttime and quiet time. Occasionally she’s up around 7 but will go back to bed herself. Now I can get more than 4-5 hours sleep to work and care for her. Benadryl or Tylenol PM are not good long term. When the doctor can work with family for solutions it’s an all around blessing. I go to all her appts so Dr gets the full/real story. I’ll say “we” do this or that. I never say her or mom; then she catches she’s being talked about and gets angry, agitated but Dr can see what’s happening so good he sees that side too. Good luck.

It may be time to consider hiring an aide to come in and be on night watch, or perhaps a Memory Care home for her. It will only get worse, not better, unfortunately. 😩 The best thing we’ve done is to separate my MIL and FIL. He was getting where he was exhausted and he was micromanaging her and she was getting angry. When she started her night roaming it became unmanageable. My husband and I both have demanding full time jobs so us taking a night shift was out of the question. She is so much calmer, we sleep better, he sleeps better, and it’s good all around.