Background: Improvements in daytime functioning ideally accompany improvements in insomnia. Scores on the Insomnia Severity Index (ISI) daytime-related items were analyzed following treatment with lemborexant (LEM), a dual orexin receptor antagonist, or placebo (PBO), based on baseline severity. Methods: Participants (≥18 y) with insomnia disorder in E2006-G000-303, a 12-month, randomized, double-blind, PBO-controlled study (first 6 months: Treatment Period 1 [TP1]), were randomized to PBO or LEM 5 mg (LEM5) or 10 mg (LEM10) for 6 months. ISI items are rated 0 (no problem) to 4 (very severe problem); daytime-related ISI items have a maximum score of 16. Results: Of 949 participants, 749 (78.9%) completed the ISI at baseline and end of TP1. Baseline daytime ISI total score distributions were similar between groups. More participants with baseline scores of 9-12 and 13-16 shifted to 0-4 with LEM5 (49.7% and 39.1%, respectively) and LEM10 (46.2% and 46.3%) versus PBO (26.6% and 29.6%). Overall shift distributions were significantly different, favoring both LEM groups (P<0.01). LEM was well tolerated. Conclusions: More LEM-treated participants had improved daytime functioning, evidenced by the significantly larger number of participants whose scores moved into lower categories (ie, better sleep) versus PBO-treated participants, demonstrating additional value beyond improved sleep parameters.