Many people with dementia are prone to sundowning which means “the emergence or worsening of neuropsychiatric symptoms (NPS) in the late afternoon or early evening.”(source) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5187352/
There are quite a few reasons for sundowning other than being overtired. With dementia, the reason is not always easy to get at. Considering that many of these people cannot truly express themselves anymore, it is up to the caregiver to try and find the reason/s why this is happening to this particular patient.
It is true that many people believe that the main causes of sundowning are issues stemming from issues involving thrown-off circadian rhythms. One way to readjust these rhythms is to bring the individual outside in the sun and allow them to relax. If done consistently, this type of therapy can have a great effect on realigning the circadian rhythms and allowing the patient to reset their internal clocks.
Other pieces of “therapy” would be to monitor the person's food intake and pain levels.
Either of these can be the main reason why sleep is so elusive. The main issue with this process is that it may take quite a bit longer to identify what the problem actually is. If pain is the reason, the person may not be able to point out where the pain is centralized or how it is affecting them. Certain tests can be done depending on the level of pain and the solution could be as simple as administering pain medication in the early evening to help them through the night.
Food and drink may also be an issue should the person not eat their meals or if they have caffeinated beverages too close to bedtime. In most cases, the first item to be taken out of the equation should be any food or drink containing caffeine. There is no reason for a person to consistently have these types of food considering that they are also the main cause of tooth decay for elderly individuals which could lead back to the pain as talked about above.
Perhaps the best thing to do, depending on the level of lucidity, is to sit the person down and simply discuss their problem with them. In cases like this, it can be quite hard to figure out what the problem might be but, because it is night time there may be more time to dedicate to the person. In skilled nursing facilities, nurses are encouraged to start and continue the conversation with the patient in an effort to allow them to reach a conclusion. If a caregiver is given enough time to think and they understand the individual an answer may surface sooner than later.
Overall, sundowning may be more of a short-term matter and one that can be solved in myriad ways. It does help to know the various avenues afforded to you to help the person get the rest they require.