and circadian resetting responses were best fit by light exposure wavelengths at 481 nm and 483 nm, respectively.
Melatonin suppression showed an initial peak sensitivity at 441 nm, indicating strong contributions from short-wavelength and long/medium-wavelength cones.
Sensitivity to melatonin suppression shifted to 485 nm by the final quarter of a 6.5-hour light exposure, suggesting a dominant role of melanopsin.
Circadian resetting responses varied from 445 nm to 487 nm, indicating significant contributions from short-wavelength cones in the initial moments of light exposure.
The findings suggest an initial reliance on visual photoreceptors that transitions to a reliance on melanopsin with longer light durations.
Simplified
Human circadian, neuroendocrine, and neurobehavioral responses to light are mediated primarily by melanopsin-containing intrinsically-photosensitive retinal ganglion cells (ipRGCs) but they also receive input from visual photoreceptors. Relative photoreceptor contributions are irradiance- and duration-dependent but results for long-duration light exposures are limited. We constructed irradiance-response curves and action spectra for and circadian resetting responses in participants exposed to 6.5-h monochromatic 420, 460, 480, 507, 555, or 620 nm light exposures initiated near the onset of nocturnal melatonin secretion. Melatonin suppression and phase resetting action spectra were best fit by a single-opsin template with lambdaat 481 and 483 nm, respectively. Linear combinations of melanopsin (ipRGC), short-wavelength (S) cone, and combined long- and medium-wavelength (L+M) cone functions were also fit and compared. For melatonin suppression, lambdawas 441 nm in the first quarter of the 6.5-h exposure with a second peak at 550 nm, suggesting strong initial S and L+M cone contribution. This contribution decayed over time; lambdawas 485 nm in the final quarter of light exposure, consistent with a predominant melanopsin contribution. Similarly, for circadian resetting, lambdaranged from 445 nm (all three functions) to 487 nm (L+M-cone and melanopsin functions only), suggesting significant S-cone contribution, consistent with recent model findings that the first few minutes of a light exposure drive the majority of the phase resetting response. These findings suggest a possible initial strong cone contribution in driving melatonin suppression and phase resetting, followed by a dominant melanopsin contribution over longer duration light exposures. max max max max
Key numbers
441 nm
Peak Sensitivity
Peak sensitivity during the first quarter of light exposure.
485 nm
Peak Sensitivity
Peak sensitivity for during light exposure.
2.52 × 10 to 1.53 × 10 photons/cm/sec
Irradiance Response Increase
range for light exposure during the study.
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The authors declare no competing interest directly related to the work presented in this manuscript. A full list of potential competing interests has been provided in the SI Appendix.
