Effects of Duration, Intensity, and Spectrum of Light Exposure on Sexual Maturation Time of Female Rats1, 2

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Abstract
The present study was undertaken to evaluate the effects of daily duration, intensity, and spectral composition of light exposure on the sexual maturation of female rats. Pregnant rats (Holtzman) delivered their young in a controlled environment with various illuminations of defined duration, intensity, and spectral composition. Pups were weaned at 22 days of age and separated by sex. Only females were used in the present study. They were checked for vaginal opening twice daily starting at 29 days of age. A group of 5-6 rats was sacrificed from each experimental condition 2-3 days prior to and after the expected date of vaginal opening. Results indicate that exposure to either 22 or 24 h of light/day significantly advanced vaginal opening when compared to 14-h/day exposure. No difference was obtained between animals in 6- and 14-h/day exposure. Light intensity was a significant variable at 14-h/day exposure, but not at 24 h/day. Animals exposed to 14 h of light at 100 lm/m2 matured faster than those at 30 or 600 lm/m2, but not significantly earlier than those at 2400 lm/m2. Animals in blue light exhibited vaginal opening significantly earlier than in red light if exposure was provided for 14 h/day. In continuous exposure, red light was more stimulating. No significant differences between yellow and green light were detected either with 14- or 24-h/day exposure, except for a small population of rats in continuous green light in which vaginal opening was significantly delayed. A significant seasonal variation was also detected in animals raised in white fluorescent light of 14-h/day exposure. Rats raised in the summer months showed vaginal opening significantly earlier than those in the winter.

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