Light reduces night-time melatonin in rats differently depending on color and time of day

Green light (520 nm) suppressed melatonin levels for at least 2 hours in Wistar and Long-Evans rats.

• Pineal and plasma melatonin levels were measured during two different phases of darkness.
• A green light pulse at midnight significantly reduced melatonin to daytime levels for at least 2 hours.
• A red light pulse at the same time only transiently suppressed pineal melatonin and did not affect plasma melatonin.
• Both green and red light pulses given at 4.00 h suppressed melatonin to a similar extent.
• The findings indicate that melatonin suppression is influenced by light wavelength and the timing within the circadian cycle.

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