No evidence for extraocular light induced phase shifting of human melatonin, cortisol and thyrotropin rhythms

The view that light affects the mammalian circadian clock only through the eyes was recently challenged by a study in which the phases of human circad ian rhythms were shifted by extraocular light exposure. This finding has no t been confirmed, however. We studied the effects of light exposure (3 h, b road spectrum fluorescent white light, 13 000 lux) on abdomen and chest on the circadian rhythms of serum melatonin, cortisol and thyrotropin in six s ubjects. The protocol consisted of two 3-day sessions in a dimly lit (< 10 lux) experimental unit. In both sessions hourly serum samples were collecte d for hormone analysis on days 1 and 3. The skin light exposure was deliver ed on day 2 from 22.00 to 01.00h in one of the two sessions in a randomized order. In both sessions all three rhythms tended to delay, presumably due to the endogenous circadian cycle length being slightly longer than 24 h. H owever, the phase shifts did not differ significantly between the sessions. Thus, the present study does not support the existence of extraocular phot ic regulation of the circadian rhythms in humans. NeuroReport 11:713-717 (C ) 2000 Lippincott Williams & Wilkins.