Mid–wavelength Monochromatic Light Is More Effective For Suppressing Plasma Melatonin In Humans Than Broadband White Light

Abstract

Abstract: : Purpose: Light responses in the circadian system are the main driving force that keeps the internal circadian clock synchronized to the external daily light–dark cycle. Assessing the sensitivity to light at different wavelengths is useful for the identification of the photoreceptor type(s) involved in transmitting light information to the internal circadian clock (located in the suprachiasmatic nucleus). Methods: Nocturnal plasma melatonin levels are acutely suppressed by light and it is likely that the same irradiance detection mechanisms driving light responses in the circadian system also suppress nocturnal melatonin secetion. We assessed the sensitivity of light induced melatonin suppression by measuring the effect of equal photon density monochromatic light (3.16 x 1012 pphotons/cm2/sec) at 9 different wavelengths spread over the visual spectrum (420 – 620 nm) in 5 healthy male subjects. Plasma melatonin levels were measured using a RIA assay in samples collected before, during, and after 1–h nocturnal light stimulation. Melatonin suppression was calculated by comparing the area under the curve of plasma melatonin values during the light stimulation with the values obtained at the same time during the dark control sessions. Results: The results indicate an optimal sensitivity to wavelengths between 460–500 nm, whereas wavelengths below 460 and above 500 nm are much less effective in suppressing plasma melatonin. At 460–500 nm, the suppression of melatonin is 3% to 7% greater than that observed with broadband white light (400–700 nm) illumination. This result is striking since the monochromatic light stimuli contain less than 2% of the total number of photons of the broadband stimulus. Conclusions:This study shows that monochromatic mid–wavelength light is more efficient than broad band white light for circadian photoreception. The results are relevant for the design of light stimulation strategies for use in shitwork and in the treatment of circadian and sleep disorders.

Keywords: circadian rhythms • melatonin • color pigments and opsins