Tehran University of Medical Sciences Journal of Sleep Sciences 2476-2938 4 3-4 2020 07 15 76 80 Reza Erfanian Otorhinolaryngology Research Center, Tehran University of Medical Sciences, Tehran, Iran Ehsan Bastanhagh Department of Anesthesiology, Pain and Critical Care, Yas Hospital, Tehran University of Medical Sciences, Tehran, Iran Khosro Sadeghniiat-Haghighi Occupational Sleep Research Center, Baharloo Hospital, Tehran University of Medical Sciences, Tehran, Iran Hakima Abdullah Otorhinolaryngology Research Center, Tehran University of Medical Sciences, Tehran, Iran 2019 09 28 2019 11 27 Background and Objective: Previous studies had examined the effect of learning in young adults with generally worse results in cases with the delayed sleep-wake phase disorder. In this study, the relationship between circadian rhythm preferences and objective structured clinical examination (OSCE) results was examined in medical students. Materials and Methods: This study was conducted during the OSCE for clinical license in medical students who were graduation candidates. The reduced form of the Horne and Ostberg Morningness-Eveningness Questionnaire was used to estimate their circadian chronotype. The OSCE consisted of 12 stations, which evaluated different clinical skills, as well as participant’s medical knowledge. The effect of morningness-eveningness preference of each participant on score was evaluated within each station of OSCE. Results: In total, 78 students participated out of a total of 114 clinically competent candidates (aged 25-34 years) were evaluated. In terms of sex, 56% of the students were girl, who obtained significantly higher grades in three stations (P < 0.05). The grades of students with delayed sleep-wake chronotypes were higher in most of the stations; however, the difference was statistically significant in only one station (P = 0.045), which was the most difficult station of the exam. Nevertheless, the delayed sleep-wake chronotype was associated with excessive daytime sleepiness (P = 0.002). Conclusion: Contrary to other similar studies in medical students, normal circadian chronotype was associated with poorer outcome in the OSCE; which may be explained by unusual student training hours during shifts, and the resultant available time for undisturbed study . https://jss.tums.ac.ir/index.php/jss/article/view/148