This is a long-term (approximately 1 year), multicenter, randomized, controlled, double-blind, parallel group study of two doses of lemborexant and placebo in approximately 900 male or female participants with insomnia disorder. Approximately 40% of participants will be age 65 years or older. The study will last a maximum of 60 weeks, and will include a Screening Period, an approximately 54-week Treatment Period (during which study medication will be administered), and a 2-week Follow-up Period. All participants will receive lemborexant for at least 6 months and will receive placebo at some point during the study. Participants will not know which medication they receive (lemborexant or placebo), and will not know the timings at which the medication will change.