Reduction in pineal N-acetyltransferase activity and pineal and serum melatonin levels in rats after their exposure to red light at night.

Pineal gland N-acetyltransferase (NAT) activity and pineal and serum levels of melatonin declined linearly in albino rats exposed to different irradiances (low, 170 microW/cm2; moderate, 420 microW/cm2; high, 1040 microW/cm2) red light during the middle of the night. High intensity red light (1040 microW/cm2) was as effective as white light (670 microW/cm2) in suppressing pineal NAT activity and pineal and serum melatonin levels. The lowered melatonin levels and the reduction in NAT activity following exposure to red light suggest that red light cannot be regarded as 'safe' light when studying circadian melatonin production in the albino rat, at least at the intensities used in this experiment.