EXTRAOCULAR LIGHT EXPOSURE DOES NOT SUPPRESS PLASMA MELATONIN IN HUMANS

Light affects the circadian axis in at least two ways. It can cause th e acute suppression of pineal melatonin synthesis, and/or a phase-shif t of the circadian oscillator. As recent evidence has suggested that e xtraocular light exposure may cause phase-shifts of the circadian cloc k, we have investigated whether suppression of melatonin can be induce d by the same type of light exposure. In the first study subjects' eye s were exposed to white light (2250 lux for 30 mins) via a fibre optic cable. As expected, suppression of nighttime plasma melatonin levels (61 +/- 6%) was observed. In the second study, light of the same quali ty but higher intensity (14,000 or 67,500 lux for 180 mins) was delive red in the same manner to the popliteal region behind the subjects' kn ees, whilst shielding their eyes. No suppression of plasma melatonin l evels (4 +/- 7%) was detected in any of the subjects. Thus, extraocula r photoreception, if it exists in mammals, does not affect the suprach iasmatic nuclei-pineal pathway.