INTRODUCTION Diurnal preference is a circadian typology consisting of three chronotypes-morning, evening, and neither types. Morningtype individuals sleep and wake up early and achieve most of their peak mental and physical performance in the early part of the day. In contrast, evening-type individuals sleep and wake up late, showing their best performance mostly during the evening hours [1] . According to many studies, compared to morning-type individuals, evening type individuals have more

more »

... ative health consequences, such as mood disorders, anxiety disorders, substance use disorders, autism, personality disorders, insomnia, sleep apnea, hypertension, diabetes, and infertility [2] . Diurnal preference has interpersonal variations due to differences in age, sex, and genetic and environmental factors that could influence circadian rhythms [3, 4] . Objective: Recent studies have extended the study of diurnal preferences to the genetic level. Most studies have focused on clock genes, but some studies have searched for the possibilities of other genes associated with the circadian rhythm. Considering that the circadian rhythm is associated with the serotonergic system, investigating the association between major genes of the serotonin system and the diurnal preference phenotype is essential. In this study, we evaluated whether the 5-HTTLPR polymorphism is associated with diurnal preference in a Korean population. Methods: In total, 509 healthy subjects were genotyped for the 5-HTTLPR polymorphism. The Korean version of the Composite Scale of Morningness (CSM) was used to measure the phenotype patterns of diurnal preference. In addition, scores of three subscales-morningness, activity planning, and morning alertness-were extracted from the CSM. Results: No significant associations were observed between CSM scores and the 5-HTTLPR genotype or allele carrier status. Conclusion: The results of this study suggests that 5-HT-TLPR has no effect on diurnal preference in a healthy Korean population. Further studies with a large number of subjects from multiple ethnicities are necessary to fully evaluate the association between 5-HTTLPR and diurnal preference.