Even so, with these limitations in mind, one potential explanation would be the existence of a higher prevalence of individuals with preclinical AD among those with insomnia, based on previous evidence of increased caudate size in presymptomatic PSEN1 mutation carriers, which is a genetic cause of AD [68], and previous findings suggest a transient size increase in some brain structures during early stages of AD [69, 70]. Here, PSEN1 is linked to Alzheimer disease.