Iris color and day–night changes in the sympathovagal ratio

Iris melanocytes are innervated by parasympathetic and sympathetic nerve endings. Light affects autonomic nervous system activity via the retino-hypothalamic pathway. The hypothesis that the day-to-night variations in the sympatovagal ratio (LF/HF) may differ among individuals with different brown iris patterns was tested. A total of 621 healthy adults, aged between 16 and 50, with brown eyes and not diagnosed with a disease that might affect the autonomous nerve system were included in the study. A digital camera was used to acquire iris photos. Subjects were grouped into iris color groups (2–0 bg, 1–0 bg, 1–1 db, 1–1 lb, 2–0 b, and 1–0 b). Iris photos were analyzed with Picture Color Analyzer RBG software. The Central/Peripheral (R/RGB) ratio was used for objective distinction between the groups. Using 24-h Holter ECG monitoring, the change in the sympathovagal ratio from day (between 07:00 and 23:00 h) to night (between 23:00 and 07:00 h) was determined with the formula [(Day–Night) LF/HF)/Day LF/HF]. The frequency of subjects with a decrease in the LF/HF ratio from day to night was the highest in the 1–1 db group (65.7%), followed by the 1–1 lb group (56.4%). The highest increase was in the 2–0 bg group (76.5%), followed by the 1–0 B group (68.9%) (p < 0.001). Based on the findings of this study, iris color may be a predictive factor in diseases in which the circadian change of autonomic nervous system activity is effective.