Light Entrainment of Rat and Human Circadian Melatonin Rhythms

The rhythmic production of melatonin in rats is driven by a circadian rhythm of the pineal N-acetyltransferase activity. The present study compares light entrainment of the rat pineal N-acetyltransferase rhythm with that of the human serum or saliva melatonin rhythm. Exposure to bright light in the early night phase delays the rat NAT rhythm and the human melatonin rhythm as a whole within one day. Exposure to bright light in the late night phase advances the morning rat NAT and human melatonin offset within one day as well; the evening rat NAT onset is, however, not phase shifted at all, whereas the human melatonin onset may or may not be phase advanced. It appears that even in humans the evening melatonin production onset does not necessarily phase shift in parallel with the morning offset. Both the rat NAT rhythm and the human melatonin rhythm are photoperiod dependent; adjustment of the rhythms to a change from long to short days may, however, proceed in a different way in rats than in humans. Altogether, though light entrainment of the circadian rhythm in melatonin production in humans is similar to that in rats, some differences exist as well.