The relationship between salivary and urinary melatonin in young adults with myopia (short-sightedness) compared to normally sighted young adults

Objectives/Introduction: Myopia or short-sightedness is an increasing epidemic that can predispose the eye to vision threatening conditions. A recent, first-of-its-kind study found a significant positive association between morning serum melatonin concentration and the magnitude of myopia, with myopes demonstrating greater serum melatonin concentration than emmetropes (normally sighted individuals). The findings indicate the potential role of melatonin levels in modulating myopia development. However, single timepoint, cross-sectional melatonin profile analyses were used that can only represent the relapsing or descending phase of the melatonin curve and not the differences in total melatonin production between myopes and emmetropes. This study aimed to comprehensively investigate the melatonin serum concentration differences varied between myopes and emmetropes. Methods: Eighteen myopic (aged [mean ± standard deviation] 22.9 ± 2.6 years) and 14 emmetropic young adults (aged 21.1 ± 1.6 years) completed online questionnaires and underwent a comprehensive eye exam to confirm eligibility. Circadian timing was assessed using salivary dim light melatonin onset, collected during one overnight in a dimly lit sleep laboratory (< 10 lux). Participants provided half-hourly saliva samples for 7 hours, beginning 5 hours before and finishing 2 hours after individual average sleep onset. Total melatonin production was assessed via aMT6s levels from urine voids collected from 6 pm and until wake-up time the following morning, when the final void was made. Results: Myopes (22:09 ± 1.8 hrs) exhibited a dim light melatonin onset phase-delay of 1 hr 9 min compared to emmetropes (21:00 ± 1.4 hrs), p = 0.03, d = 0.71. Urinary aMT6s melatonin levels were significantly lower among myopes (29.17 ± 18.67) than emmetropes (42.51 ± 23.97, p = 0.04, d = 0.63). Conclusions: Young adult myopes have significantly delayed circadian rhythm timing than normal sighted emmetropes. Myopes also have lower melatonin output, delayed and reduced sleep, and evening diurnal preference compared to emmetropes. These findings indicate a potentially important role of circadian rhythms in refractive error development.