Gathered from the web. Hope it helps.
http://what-when-how.com/parkinsons-disease/sun...

.Episodes of delusions, hallucinations, agitation, and increased dementia that intensify in the evening. clinicians do not fully understand the timing of sundowning episodes. Theories include correlations with hormonal cycles in the body, fluctuations in brain neurotransmitter levels that result from medication peaks and troughs, a “crash” letdown+ from the activities of the day, and the somewhat surrealistic images that can occur as a combination of artificial lighting and shadows as evening begins. As are other dimensions of neurodegenerative diseases, it is likely a combination of all of these factors.m

http://www.parkinsonsresource.org/education/wha...
Sundowning, by its very name, occurs at a specific time of day, generally starting in the late afternoon and lasting until bedtime. It is thought to be associated with impaired circadian rhythmicity, environmental and social factors, and impaired cognition.
Sundowning can be exhibited in any or all of the following examples: Agitation, anxiety, confusion, combativeness, ignoring directions, irritability, restlessness, and screaming or yelling and may even manifest in constant pacing and/or wandering. Some of these behaviors may not be specific to Sundowning and can be the manifestation of dementia, delirium, Parkinson’s disease, and sleep disturbances

My dad had sundowners with his Alzheimer's it when they get confused with day and night

It was very helpful, thank you.