A variety of epidemiological and genetic studies have revealed that disturbances of circadian rhythms can be directly linked to the risk of cancer development [1, 2] . This is because cell cycle regulators, which are essential for cell proliferation and at the same time closely associated with tumor production, are under the control of the circadian clock. Interestingly, in the case of rapidly dividing cells, such as tumor cells or stem cells, the expression of the biological clock genes is

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... ificantly suppressed. This early study results have raised the possibility that the activation of the circadian biological clock plays a key role in tumor production and its treatment. In particular, since the study of the biological clock was conducted in earnest, it has been known that the efficacy of anticancer drugs can be maximized and the side effects on normal cells Objective: Recent epidemiological and clinical studies have revealed that timing medications to the individual's biological clock could improve their effectiveness and reduce side effects. We aimed to investigate the association of escitalopram dosing time with treatment response and changes in global depressive and insomnia symptoms after 8-week of antidepressant treatment in a naturalistic setting in patients with major depressive disorder (MDD). Methods: A total of 71 patients with MDD taking escitalopram were recruited from the outpatient psychiatric clinic in a university hospital. The severity of depressive symptoms of all patients was assessed using the 17-item Hamilton Depression Rating Scale (HDRS) at the baseline and 8-week after the antidepressant treatment. The patients were classified into the Morning or Evening/Night group according to their time of escitalopram single dose. Results: The time of escitalopram single dose (morning vs. evening/night) was not associated with treatment response after 8-week of treatment when controlling for potential confounding factors in patients with MDD. No significant difference in changes in total HDRS score and insomnia-related subscale score was observed between the Morning and Evening/ Night groups. Conclusion: The present study suggests that the time of escitalopram single dose did not affect the treatment response or changes in global depressive and insomnia symptoms after 8-week of antidepressant treatment in a naturalistic setting.