Light suppression of nocturnal pineal and plasma melatonin in rats depends on wavelength and time of day.

Effects of light on the pineal and plasma melatonin were examined in Wistar and Long-Evans rats at two different times in the dark phase (light off from 18.00 h to 06.00 h) using lights of two different monochromatic wavelengths but with the same irradiance. The green light pulse (520 nm) given at 24.00 h suppressed the pineal and plasma melatonin to the day-time level for at least 2 h, while the red light (660 nm) pulse given at the same time of the day suppressed pineal melatonin only transiently and did not suppress the plasma melatonin at all. Both green and red lights given at 4.00 h suppressed the pineal and plasma melatonin to a similar extent. The results demonstrated that the suppression of melatonin by light depends on the wavelength of light and the circadian phase.