Elevated Melatonin Levels Found in Young Myopic Adults Are Not Attributable to a Shift in Circadian Phase.

Purpose: To evaluate the relationship between refractive error, circadian phase, and melatonin with consideration of prior light exposure, physical activity, and sleep. Methods: Healthy young myopic (spherical equivalent refraction [SER] ≤-0.50DS) and emmetropic adults underwent noncycloplegic autorefraction and axial length (AL) measures. Objective measurements of light exposure, physical activity, and sleep were captured across 7 days by wrist-worn Actiwatch-2 devices. Questionnaires assessed sleep quality and chronotype. Hourly evening saliva sampling during a dim-light melatonin onset (DLMO) protocol evaluated circadian phase, and both morning serum and saliva samples were collected. Liquid chromatography/mass spectrometry quantified melatonin. Results: Subjects (n = 51) were aged 21.4 (interquartile range, 20.1-24.0) years. Melatonin was significantly higher in the myopic group at every evening time point and with both morning serum and saliva sampling (P ≤ 0.001 for all). DLMO-derived circadian phase did not differ between groups (P = 0.98). Multiple linear regression analysis demonstrated significant associations between serum melatonin and SER (B = -.34, β = -.42, P = 0.001), moderate activity (B = .009, β = .32, P = 0.01), and mesopic illumination (B = -.007, β = -.29, P = 0.02), F(3, 46) = 7.23, P < 0.001, R2 = 0.32, R2adjusted = .28. Myopes spent significantly more time exposed to "indoor" photopic illumination (3 to ≤1000 lux; P = 0.05), but "indoor" photopic illumination was not associated with SER, AL, or melatonin, and neither sleep, physical activity, nor any other light exposure metric differed significantly between groups (P > 0.05 for all). Conclusions: While circadian phase is aligned in adult myopes and emmetropes, myopia is associated with both elevated serum and salivary melatonin levels. Prospective studies are required to ascertain whether elevated melatonin levels occur before, during, or after myopia development.