St Hilaire · Proceedings of the National Academy of Sciences of the United States of America 2022 · controlled laboratory experimental study · n=?

The spectral sensitivity of human circadian phase resetting and melatonin suppression to light changes dynamically with light duration.

Level 3 - non-randomized controlled study

Controlled human laboratory photobiology experiment without reported randomization details in the abstract.

PubMed 36508661 · doi:10.1073/pnas.2205301119 · record verified 2026-08-26

What was done

Participants were exposed near nocturnal melatonin onset to 6.5-hour monochromatic light at one of six wavelengths (420, 460, 480, 507, 555, or 620 nm). Researchers evaluated melatonin suppression and circadian phase resetting to construct irradiance-response curves and action spectra, fitting data to single-opsin templates as well as combinations of melanopsin (ipRGC), short-wavelength (S) cone, and long/medium-wavelength (L+M) cone spectral functions across different time bins.

What was found

Overall 6.5-hour action spectra peaked at 481 nm for melatonin suppression and 483 nm for phase resetting using a single-opsin fit. For melatonin suppression, spectral sensitivity shifted dynamically: peak sensitivity was 441 nm (with a secondary peak at 550 nm) during the first quarter of the exposure, reflecting strong initial S-cone and L+M-cone contributions, and shifted to 485 nm by the final quarter, indicating melanopsin predominance. For phase resetting, peak sensitivity ranged between 445 nm and 487 nm depending on the photoreceptor model fit. Participant numbers, confidence intervals, and absolute effect sizes were not reported in the abstract.

Why it matters

This study shows that human circadian phase resetting and melatonin suppression rely on a dynamic handoff from visual cones during acute light onset to melanopsin over sustained exposures. These dynamics are critical for designing accurate circadian lighting metrics and therapeutic interventions.

Limits

The abstract does not disclose participant sample size, age, sex, or whether conditions were randomized or counterbalanced. Findings from nocturnal monochromatic exposures may not directly generalize to polychromatic, broad-spectrum daytime lighting.