“Sundowning” is the state of agitation, confusion, restlessness or delirium observed in the afternoon, evening, or night with symptoms improving or disappearing during the day. A higher percentage of nocturnal activity, lower interdaily stability of motor activity, and a later activity acrophase (time of peak) may be noted. Often associated with dementing disorders such as Alzheimer’s disease.
Goldstein, Rachel, Harper, David, G, Manning, Barbara C, Satlin, Andrew, and Volicer, Ladislav . 2001. Sundowning and Circadian Rhythms in Alzheimer’s Disease. The American Journal of Psychiatry. 158: 704-711.