This study aimed to examine differences in salivary melatonin concentrations between the sitting and standing positions. Ten subjects (age range: 20–22 years; 5 males and 5 females) participated in the study and provided written consent. In the sitting condition, the subjects sat on a chair from 23:30？h to 1:00？h, whereas in the standing condition, the subjects sat on a chair from 23:30？h to 0:30？h and stood on the floor from 0:30？h to 1:00？h. Saliva samples were obtained at 0:25？h and 1:00？h from all subjects. The experiment was performed under a dim light condition (<10？lx). No significant differences were observed in melatonin concentrations between the two positions at 0:25？h; however, melatonin concentrations in the standing condition were significantly greater than those in the sitting condition at 1:00？h. These results show that melatonin secretion is significantly greater in the standing position than that in the sitting position. 1. Background Melatonin, a hormone secreted by the pineal gland, is measured during human circadian rhythm studies because the secretion is controlled by the light-dark signal from the suprachiasmatic nucleus (SCN). The SCN, which is responsible for circadian rhythms, maintains the melatonin secretion, body temperature, and other hormones such as cortisol. Melatonin exhibits a 24？h secretion pattern, and its concentration is high at night and low during the day. The melatonin secretion pattern, particularly dim light melatonin onset, is used as a circadian phase marker [1]. Furthermore, melatonin secretion is acutely suppressed by bright light [2]. Light-induced melatonin suppression shows a dose-response relationship depending on light intensity [3, 4] and duration of light exposure [5]. Melatonin secretion is less susceptible to behavioral and environmental masking factors compared with other circadian rhythm markers such as core temperature and cortisol levels. However, previous studies have reported that postural change from a supine to a standing position significantly increases the melatonin secretion [6, 7]. A study [7] revealed no significant difference in plasma melatonin level with postural change from a sitting to a standing position. Many recent studies have evaluated salivary melatonin concentration because it exhibits a good correlation with blood melatonin concentration [8]; furthermore, collecting saliva samples is less invasive. In this study, we examined the differences in salivary melatonin concentrations between the subjects in standing and sitting positions. 2. Methods 2.1. Subjects The study
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