Bedrosian TA, Herring KL, Weil ZM, Nelson RJ.
Altered temporal patterns of anxiety in aged and amyloid precursor protein (APP) transgenic mice.Proc Natl Acad Sci U S A.
2011 Jul 12;108(28):11686-91.
As a practitioner with years of experience caring for clients with AD, I can say that sundowning is definitely a biological phenomenon. This is an important area of research, at least in terms of symptom management.