VIP and Alzheimer disease: One proposed explanation of circadian dysfunction in AD is the Aβ-related impairment of the SCN caused by the drastic loss of vasopressin- and vasoactive intestinal peptide-expressing neurons which are essential in the maintenance of the SCN circadian function (Swaab et al., 1985; Zhou et al., 1995; Farajnia et al., 2012).