Submitted on May 26, 2006
Accepted on February 21, 2007
Influence of eye colors of Caucasians and Asians on suppression of melatonin secretion by light
Shigekazu Higuchi1*, Yutaka Motohashi2, Keita Ishibashi3, and Takafumi Maeda4
1 Japan National Institute of Occupational Safety and Health, Kawasaki, Kanagawa, Japan
2 Department of Public Health, Akita University School of Medicine, Akita, AKITA, Japan
3 Department of Human Living System Design, Kyushu University, Fukuoka, Fukuoka, Japan
4 Laboratory of Environmental Ergonomics, Hokkaido University, Sapporo, Fukushima, Japan
* To whom correspondence should be addressed. E-mail: higuchi{at}h.jniosh.go.jp.
This experiment tested effects of human eye pigmentation depending on the ethnic on suppression of nocturnal melatonin secretion by light. Ten healthy Caucasian males with blue, green or light brown irises (light-eyed Caucasians) and eleven 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 (1000 lx) for two hours at night. The starting time of exposure was set to two hours 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 two hours after the start of bright 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.
