EFFECTS OF DAYTIME MELATONIN INFUSION IN YOUNG-ADULTS

Daytime oral melatonin typically exerts soporific and thermoregulatory effects; however, it is not clear whether these effects reflect the n ormal physiological response to endogenous nocturnal melatonin product ion. We infused melatonin at doses that produced physiological and sup raphysiological steady-state levels in 24 young adults during two dayt ime bed rest protocols. From 1000 to 1630, subjects were infused intra venously with saline or melatonin in counterbalanced order. Each group of eight subjects received melatonin (and saline) infusions at one do se rate: 0.04 mu g.h(-1).kg body wt(-1) (low), 0.08 mu g.h(-1).kg(-1) (medium), or 8.0 mu g.h(-1).kg(-1) (high). Low and medium melatonin in fusions produced plasma and saliva levels within the normal nocturnal range observed in young adults. These levels were not associated with any changes in rectal, hand, forehead, or tympanic temperatures or wit h subjective sleepiness. High melatonin produced supraphysiological pl asma and saliva levels and was associated with a significant attenuati on in the daytime increase in rectal temperature, significantly increa sed hand temperature, and greater sleepiness. It is not yet clear whet her the thermoregulatory and soporific effects of daytime supraphysiol ogical melatonin administration are equivalent to the physiological re sponses to endogenous melatonin.