PURPOSE: The effect of mandibular advancement device therapy on daytime sleepiness remains unclear. Here, we evaluate the effect of a mandibular advancement device on daytime sleepiness using the Karolinska Sleepiness Scale. METHODS: We randomized 88 snoring patients with an apnea-hypopnea index <
30 and daytime sleepiness to a mandibular advancement device or a sham device for four months. The Karolinska Sleepiness Scale, which measures grades of sleepiness from 1 (very alert) to 9 (very sleepy), was used for seven consecutive days, four times each day. The results were analyzed with quantile regression at quartiles controlling for baseline, age, body mass index (kg/m RESULTS: The Karolinska Sleepiness Scale score was lower with the mandibular advancement device than with the sham device at specific time intervals. The positive effect of mandibular advancement device therapy occurred at wake up and before lunch during the whole week and before lunch on weekdays at the middle quartile. The adjusted differences between the interventions favored mandibular advancement device therapy by almost one unit and normalized the Karolinska Sleepiness Scale scores at wake up and before lunch. In addition, there were positive effects of mandibular advancement device therapy before dinner at the highest quartile during the whole week, on weekdays, and on the weekend. CONCLUSION: Mandibular advancement devices used for snoring and sleep apnea reduce daytime sleepiness, particularly at wake up and before lunch, but provide some benefit before dinner.