BACKGROUND. Beyond image formation, the light that is detected by retinal photoreceptors influences subcortical functions, including circadian timing, sleep, and arousal. The physiology of nonimage-forming (NIF) photoresponses in humans is not well understood; therefore, the development of therapeutic interventions based on this physiology, such as bright light therapy to treat chronobiological disorders, remains challenging.
METHODS. Thirty-nine participants were exposed to 60 minutes of either continuous light (n = 8) or sequences of 2-millisecond light flashes (n = 31) with different interstimulus intervals (ISIs; ranging from 2.5 to 240 seconds). Melatonin phase shift and suppression, along with changes in alertness and sleepiness, were assessed.
RESULTS. We determined that the human circadian system integrates flash sequences in a nonlinear fashion with a linear rise to a peak response (ISI = 7.6 ± 0.53 seconds) and a power function decrease following the peak of responsivity. At peak ISI, flashes were at least 2-fold more effective in phase delaying the circadian system as compared with exposure to equiluminous continuous light 3,800 times the duration. Flashes did not change melatonin concentrations or alertness in an ISI-dependent manner.
CONCLUSION. We have demonstrated that intermittent light is more effective than continuous light at eliciting circadian changes. These findings cast light on the phenomenology of photic integration and suggest a dichotomous retinohypothalamic network leading to circadian phase shifting and other NIF photoresponses. Further clinical trials are required to judge the practicality of light flash protocols.
(A) Melatonin concentrations (n = 8, W = 35, Z = 2.31, *P < 0.05, Wilcoxon signed-rank test) and (B) median RT (n = 8, t = 4.49, df = 7, **P < 0.01, paired t test) are significantly decreased under continuous light exposure. (C) Participants exposed to flashes who exhibit a similar circadian phase shift as those exposed to continuous light (i.e., within the 95% CI) do not exhibit a significant decrease in melatonin concentrations (n = 7, W = 12, Z = –0.25, P = 0.81, Wilcoxon signed-rank test) or (D) median RT (n = 10, t = –0.02, df = 9, P = 0.98, paired t test) after the different flash exposures. Individual data before and after light exposure are represented as white circles connected with gray bars. (A and C) Median and (B and D) average data are represented as red circles connected by black bars.