People who suffer from dementia often act differently at different times of the day. It is common for many to get worse near sundown.
If you use an alarm clock to wake up at the same time every day, you may reach a point where you do not really need the clock. Instead, you may sometimes wake up just before the alarm normally goes off. If you forget to set the alarm, you will often wake up at about the same time as well.
This can happen even if you went to bed much later than normal the previous night. People commonly refer to this phenomenon as their “biological clock” and we all have one.
For elderly people with dementia or cognitive impairment, their biological clock can affect them in another way as Caring.com discussed in "How to Deal With Signs of Sundown Syndrome."{: target="_blank"}
Dementia can cause a person's biological clock to change. A common change? Becoming agitated or to suffer from other worsening symptoms at about the time the sun sets. This is called sundown syndrome. Elderly people and their caregivers should be aware of this problem, so any associated issues can be addressed.
The source of any increased agitation should be examined. For example, if there is a particular thing that upsets a person suffering from dementia about sundown that can be changed.
It might help if meals were served earlier. Another simple thing that has been proven to help, is to turn on a full spectrum lamp, since it replicates sunlight better than regular lightbulbs.
Reference: Caring.com (February 26, 2018) "How to Deal With Signs of Sundown Syndrome."{: target="_blank"}