SUPPRESSION OF MENS RESPONSES TO SEASONAL-CHANGES IN DAY LENGTH BY MODERN ARTIFICIAL LIGHTING

We recently reported that humans have conserved mechanisms, like those that exist in other animals, which detect changes in day length and m ake corresponding adjustments in the duration of nocturnal periods of secretion of melatonin and of other functions. We detected these respo nses in individuals who were exposed to artificial ''days'' of differe nt durations. The purpose of the present study was to determine whethe r men who are exposed to natural and artificial light in an urban envi ronment at 39 degrees N are still able to detect and respond to season al changes in duration of the natural photoperiod. We measured profile s of circadian rhythms during 24-h periods of constant darkness (<1 Ix ) and found no summer-winter differences in durations of nocturnal per iods of active secretion of melatonin, rising levels of cortisol, high levels of thyrotropin, and low levels of rectal temperature. The resu lts of this and our previous study suggest that modern men's use of ar tificial light suppresses responses to seasonal changes in the natural photoperiod that might otherwise occur at this latitude.