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Abstract The melanopsin-containing intrinsically photosensitive ipRGCs responsible for the synchronisation of the circadian clock with the environmental light-dark cycle are characterised by a delayed off-time following cessation of light exposure. In this work, we exploited this unusual physiologic property and interrogated how a sequence of flashes of bright light differing in duration could delay the human circadian clock in healthy young participants (n=27). Surprisingly, a sequence of 10 μ s flashes (2,000 lux) delivered over the course of one hour leads to a phase shift equivalent to that observed after exposure to the same number of 10 s flashes at (2,000 lux), with no parametric relationship for intermediate flash durations. Our result demonstrates that the human circadian system can respond to light in a duration-invariant manner at very short durations, over six orders of magnitude, suggesting circadian responses to ultra-short flashes (sub-second) of light are saturated with even moderate photopic light levels.

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