Growing evidence has suggested an association between sleep duration and Alzheimer's disease (AD), but it is unclear if sleep duration is a manifestation of AD disease process. We studied whether genetic liability for AD predicts sleep duration using a genetic risk score for AD (AD-GRS), in 406,536 UK Biobank participants with European ancestry and without dementia at enrollment. Higher AD-GRS score was associated with shorter sleep (b= -0.014,95%CI:-0.022,-0.006), especially in those aged 55+. Using AD-GRS as an instrumental variable for AD diagnosis, incipient AD reduced sleep duration by 1.87 hours (95%CI:0.96, 2.78). Short sleep duration might be an early marker of AD. This article is protected by copyright. All rights reserved.