The Suppression of Nocturnal Pineal Melatonin in the Syrian Hamster: Dose-Response Curves at 500 and 360 nm*

The calculated ED50 values for suppressing pineal melatonin in hamsters were 0.022 microW/cm2 for 500 nm light and 0.306 microW/cm2 for 360 nm light.

• Both 500 nm and 360 nm light suppressed pineal melatonin in a dose-related manner.
• The fluence-response curves for both wavelengths were similar in shape but had different threshold irradiances.
• Inducing a 50% decrease in pineal melatonin required 10 times the number of photons from 360 nm compared to 500 nm.
• Environmental levels of UV-A light are significantly higher than the thresholds needed for melatonin suppression in hamsters.
• These findings highlight the role of UV-A light in regulating pineal physiology alongside visible light.

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