PUPIL SIZE REGULATION OF THRESHOLD OF LIGHT-INDUCED MELATONIN SUPPRESSION

The capacity of pupil dilation to affect light-induced plasma melatoni n suppression was tested by exposing human subjects with freely constr icting or pharmacologically dilated pupils to either 50 (n = 6), 100 ( n = 8), or 200 lux (n = 5) of white light presented over the entire vi sual field. Pupil dilation significantly enhanced low level white ligh t-induced melatonin suppression over that elicited with freely constri cting pupils. Although 100 and 200 lux white light exposures resulted in significant melatonin suppression over control (no light) condition s, the effects of 50 lux were not strong enough to demonstrate statist ically significant suppression with six subjects. Linear regression di d not reveal a systematic relationship between theoretical retinal ill uminance in Trolands and magnitude of melatonin suppression. These res ults suggest that pupil diameter may be a factor in the effectiveness of light stimuli used to shift circadian rhythms or to treat seasonal depression or sleep disorders.