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 theory that responses to light flashes are mediated by the
extrinsic, rod/cone pathway and saturate the response of this pathway
within 15 minutes. Further excitation leads to no greater change in
circadian timing and no longer can acutely suppress melatonin, indicating
that this pathway may be in a refractory state following this brief light
stimulation. As the light flash sequences can be administered while
individuals are sleeping, and without waking an individual, these short,
15-minute exposures may be especially beneficial as a clinical
intervention for a variety of circadian-based sleep disorders.