The effects of environmental lighting on the daily melatonin rhythm in primate cerebrospinal fluid

The effects of alterations in environmental lighting on the daily rhythm in cerebrospinal fluid concentrations of melatonin were studied in the rhesus monkey. It was found that acute exposure to darkness during the day did not markedly increase normally low daytime CSF melatonin levels, that light suppressed the normally high CSF melatonin values at night, and that 12-h phase shifts in the diurnal lighting cycle caused 12-h phase shifts in the rhythm. The daily rhythm persisted for 6.5 days of study in constant darkness and the phase of the rhythm was not affected in constant darkness by a 12-h phase shift in the daily delivery of food and daily care of the animals. These results support the notion that the melatonin rhythm in this primate species is endogenous in nature, and that light can act to both coordinate the rhythm to the 24-h day and to acutely suppress melatonin production.