Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor.
Level 3 - non-randomized controlled study
Non-randomized human experimental physiological dose-response study
PubMed 11487664 · doi:10.1523/JNEUROSCI.21-16-06405.2001
What was done
Healthy adults with normal color vision (n = 72; 37 females, 35 males; mean age 24.5 ± 0.3 years) received full-field monochromatic light exposures between 2:00 and 3:30 A.M. with dilated pupils. Each subject was exposed to at least seven irradiances at a single wavelength (ranging between 420 and 600 nm) with at least 1 week between sessions, yielding 627 completed nighttime trials. Blood samples were collected before and after light exposure to quantify melatonin suppression.
What was found
Fluence-response data across wavelengths fit eight univariant, sigmoidal curves (R² = 0.81–0.95). The resulting action spectrum aligned with an opsin template (R² = 0.91), establishing the 446–477 nm region as the most potent wavelength band for human melatonin suppression. The peak sensitivity was distinct from classical rod and cone visual photopigments and consistent with a retinaldehyde-based opsin.
Why it matters
This study provided foundational human physiological evidence that non-visual circadian light transduction is mediated by a distinct, short-wavelength-sensitive photoreceptor system (later characterized as melanopsin-expressing retinal ganglion cells), fundamentally changing how lighting and circadian physiology are studied.
Limits
The abstract does not state whether exposure order was randomized. Testing was conducted exclusively in young, healthy adults under artificial pupil dilation during nighttime hours, which does not capture daytime responses, natural pupil kinetics, or older populations with altered lens transmission.
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