Background: Obstructive sleep apnea (OSA) may be associated with sleep difficulties and decreased rapid eye movement (REM) sleep. Lemborexant (LEM), a dual-orexin-receptor-antagonist approved to treat adults with insomnia, increases total sleep time (TST) and REM sleep, and demonstrated respiratory safety in subjects with mild through severe OSA. Sleep architecture was thus analyzed after LEM treatment in those subjects. Methods: Studies E2006-A001-102 and E2006-A001-113 enrolled adults with mild (apnea-hypopnea index [AHI] ≥5 – <15) or moderate (AHI ≥15 – <30)/severe (AHI ≥30) OSA without insomnia. Subjects received LEM 10mg (LEM10) or placebo (PBO) in 2 treatment periods, Days 1 (D1) and 8 (D8), separated by ≥14 days. Least-squares-mean (minutes) for each sleep stage was compared. Treatment-emergent adverse events (TEAEs) were recorded. Results: Thirty-nine subjects with mild and 33 with moderate/severe OSA were randomized. On both days, TST was significantly higher in the LEM period for these subjects. Total non-REM on D1 in subjects with mild OSA and on both days in subjects with moderate/severe OSA were higher with LEM than PBO; REM also significantly increased in subjects with mild and moderate/severe OSA. Most TEAEs were mild. Conclusions: In OSA subjects without insomnia, LEM was associated with higher TST, non-REM, and REM versus PBO.