No melatonin suppression by illumination of popliteal fossae or eyelids

A recent report that popliteal illumination shifted the circadian rhythms o f body temperature and melatonin challenged the longstanding belief that li ght phase-shifting the circadian system in mammals is mediated only through the retina. The authors tested effects of popliteal illumination and illum ination provided through the eyelids on melatonin suppression. In randomize d, counterbalanced orders, healthy volunteers received three treatments fro m midnight until 2:00 AM, one on each of three visits to the laboratory. Tr eatments included (1) no illumination from light pads applied to the poplit eal fossae, with light mask maintained at < 3 lux (control); (2) light mask illuminated at 1700 lux, with popliteal light pads extinguished; and (3) p opliteal light pads illuminated (13,000 lux) and light mask at < 3 lux (con trol). Saliva specimens were sampled at midnight, at 1:00 AM, and at 2:00 A M. Mean salivary melatonin concentrations rose from an average of 30.8 (3.9 ) pg/ml at midnight (baseline), to 33.2 (4.0) pg/ml at 1:00 AM, and to 37.2 (3.8) pg/ml at 2:00 AM in all three conditions, but no statistical differe nces were found using repeated-measures ANOVA. No evidence of melatonin sup pression by either popliteal or closed eyelid light stimulation was found. These data suggest that bright retinal illumination is necessary for suppre ssion of melatonin mediated through the suprachiasmatic nuclei.