 This is a multicenter, randomized, double-blind, placebo-controlled, parallel-group study of lemborexant 4 dose levels or placebo taken daily for 4 weeks in approximately 125 male or female participants, ages 60 to 90 years, with mild or moderate Alzheimers disease dementia AD-D who complain of disrupted sleep or multiple awakenings at night along with frequent periods of falling asleep during the day that impact the quality of life of the participant. The study is designed to determine the dose response of lemborexant on the change from baseline in actigraphy Sleep Efficiency aSE and actigraphy Wake Efficiency aWE during the last week of 4 weeks of treatment in participants with Irregular Sleep-Wake Rhythm Disorder ISWRD and AD-D. 