Influence of eye colors of Caucasians and Asians on suppression of melatonin secretion by light

1 Department of Public Health, Akita University School of Medicine; 2 National Institute of Occupational Safety and Health; 3 Department of Human Living System Design, Faculty of Design, Kyushu University; and 4 Laboratory of Environmental Ergonomics, Graduate School of Engineering, Hokkaido University, Akita, Japan Submitted 26 May 2006 ; accepted in final form 21 February 2007 This experiment tested effects of human eye pigmentation depending on the ethnicity on suppression of nocturnal melatonin secretion by light. Ten healthy Caucasian males with blue, green, or light brown irises (light-eyed Caucasians) and 11 Asian males with dark brown irises (dark-eyed Asians) volunteered to participate in the study. The mean ages of the light-eyed Caucasians and dark-eyed Asians were 26.4 ± 3.2 and 25.3 ± 5.7 years, respectively. The subjects were exposed to light (1,000 lux) for 2 h at night. The starting time of exposure was set to 2 h before the time of peak salivary melatonin concentration of each subject, which was determined in a preliminary experiment. Salivary melatonin concentration and pupil size were measured before exposure to light and during exposure to light. The percentage of suppression of melatonin secretion by light was calculated. The percentage of suppression of melatonin secretion 2 h after the start of light exposure was significantly larger in light-eyed Caucasians (88.9 ± 4.2%) than in dark-eyed Asians (73.4 ± 20.0%) ( P < 0.01). No significant difference was found between pupil sizes in light-eyed Caucasians and dark-eyed Asians. These results suggest that sensitivity of melatonin to light suppression is influenced by eye pigmentation and/or ethnicity. pigmentation; retina; iris; circadian rhythm; ethnic Address for reprint requests and other correspondence: S. Higuchi, National Institute of Occupational Safety and Health, Japan, 6-21-1 Nagao, Tama-ku, Kawasaki, 214-8585 Japan (e-mail: higuchi{at}h.jniosh.go.jp )