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.

The phase shift data were obtained through calculation of salivary melatonin onset on subsequent nights, with a light intervention between the two assessments. The melatonin suppression data were obtained as the difference in acute salivary melatonin levels from immediately before and at the end of the light intervention. The change in median reaction time was obtained from a 10-minute auditory psychomotor vigilance task given immediately before and at the end of the light intervention.

The light intervention was a sequence of light flashes. Each light flash last 2 ms, was 2200 lux in illuminance, and given with 8 s of darkness between. The duration of the sequence varied as indicated, from 15 minutes to 3.5 hours. The light flash sequence was centered at 1.5 hours prior to mid-sleep (verified with post hoc melatonin phase determination). Melatonin phase was determined on a constant routine in dim light.

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