Optimizing Light Flash Sequence Duration to Shift Human Circadian Phase

Abstract

Unlike light input for forming images, non-image-forming retinal pathways are optimized to convey information about the total light environment, integrating this information over time and space. In a variety of species, discontinuous light sequences (flashes) can be effective stimuli, notably impacting circadian entrainment. In this study, we examined the extent to which this temporal integration can occur. A group of healthy, young (n = 20) individuals took part in a series of 16-day protocols in which we examined the impact of different lengths of light flash sequences on circadian timing. We find a significant phase change of −0.70 h in response to flashes that did not differ by duration; a 15-min sequence could engender as much change in circadian timing as 3.5-h sequences. Acute suppression of melatonin was also observed during short (15-min) exposures, but not in exposures over one hour in length. 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 min. Further excitation leads to no greater change in circadian timing and an inability to acutely suppress melatonin, indicating that this pathway may be in a refractory state following this brief light stimulation.

Human-Centric Lighting Research and Policy in the Melanopsin Age

“…Recently, a series of studies demonstrated that sequences of short flashes at the millisecond scale delivered over the course of 1 hour can lead to stronger circadian phase shifts than 1-hour exposure of continuous light at the same illuminance (Joyce et al, 2022a(Joyce et al, , 2022bNajjar & Zeitzer, 2016). These "circadian illusions" suggest that fewer photons can sometimes lead to a stronger physiological response, offering a novel method for maximizing circadian responses to light while minimizing energy use.…”

“…These responses likely arise from complex temporal integration of rod, cone, and ipRGC signals. While duration and intensity properties have been addressed before (Joyce et al, 2022a(Joyce et al, , 2022b, there are still many combinations of parameters to be explored.…”