The study will have 2 phases: the Prerandomization Phase and the Randomization Phase. The Prerandomization Phase will comprise 3 periods that will last up to a maximum of 35 days: a Screening Period, a Run-in Period, and a Baseline Period. The Randomization Phase will comprise a Treatment Period during which participants will be treated for 30 nights (1 month) and a minimum 14-day Follow-up Period before an End of Study (EOS) Visit (up to 54 days). The total study duration for each participant on this study is 89 days.
The primary purpose of this study is to confirm using polysomnography (PSG) that lemborexant 10 milligram (mg) is superior to placebo on objective sleep onset as assessed by latency to persistent sleep (LPS) during the last 2 nights of 1 month of treatment in participants with insomnia disorder.