Duration invariance and intensity dependence of the human circadian system phase shifting response to brief light flashes

Mar 9, 2022Proceedings. Biological sciences

Human circadian system shifts timing depending on light flash brightness but not duration

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Abstract

A dose-response relationship exists between flash intensity and circadian phase shift, with 28 participants studied at fixed duration and 31 participants at variable duration.

Circadian phase shifts were influenced by the intensity of light flashes, suggesting a potential threshold effect.
Flash duration showed a slight relationship with circadian phase shift.
No significant impact was found on melatonin suppression or measures of alertness and sleepiness from light exposure.
Responses to light flashes appear to be integrated by specialized retinal cells that process input from other photoreceptors.

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The melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) are characterized by a delayed off-time following the cessation of light stimulation. Here, we exploited this unusual physiologic property to characterize the exquisite sensitivity of the human circadian system to flashed light. In a 34 h in-laboratory between-subjects design, we examined phase shifting in response to variable-intensity (3-9500 photopic lux) flashes at fixed duration (2 ms;= 28 participants) and variable-duration (10 µs-10 s) flashes at fixed intensity (2000 photopic lux;= 31 participants). Acute melatonin suppression, objective alertness and subjective sleepiness during the flash sequence were also assessed. We find a dose-response relationship between flash intensity and circadian phase shift, with an indication of a possible threshold-like behaviour. We find a slight parametric relationship between flash duration and circadian phase shift. Consistent with prior studies, we observe no dose-response relationship to either flash intensity or duration and the acute impact of light on melatonin suppression, objective alertness or subjective sleepiness. Our findings are consistent with circadian responses to a sequence of flashes being mediated by rod or cone photoreceptors via ipRGC integration. n n

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Duration invariance and intensity dependence of the human circadian system phase shifting response to brief light flashes

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