Flash sequence duration

Unlike light input for forming images, non-image forming retinal pathways are optimized to convey information about the total light environment, integrating over time and space. It has been demonstrated in a variety of species that the retinohypothalamic pathway underlying circadian entrainment can effectively integrate the impact of individual flashes over time and convey this information as if continuous light exposure had occurred. In this study, we examine the extent to which this temporal integration can occur. A group of healthy, young (N=20) individuals took part in a series of 16-d protocols in which we examined the impact of different lengths of light flash exposure sequences on circadian timing. We find that a sequence as short as 15 minutes can engender as much change in circadian timing as sequences that last for 3.5 hours. Acute suppression of melatonin was also observed during short (15-minute) exposures, but not in exposures over one hour. Our data are consistent with the...