The effects of extraocular light on urinary 6-sulphatoxymelatonin

Abstract

Extraocular light could be a better means for light therapy. The aim of the present study was to determine whether extraocular bright light can suppress melatonin secretion and induce phase delay on the onset of melatonin secretion on the following night. The study employed a within-subject design. A single group of 48 subjects, 26 female and 22 male, was recruited. They were young and healthy adults with age range from 20-30. The participants stayed in a home-like sleep laboratory for three consecutive nights. Urine was collected hourly from 2100 h to midnight and 2-hourly from 0400 h to 0800 h for 3 nights. The experimental light, 13,000 lux of blue light, was presented behind the knee from midnight to 0400 h on night 2. Melatonin metabolite, 6-Sulphatoxymelatonin (aMT6s), was measured by Radioimmunoassay (RIA). Both aMT6s levels and aMT6s concentration adjusted with creatinine were calculated for data analysis. Analysis of variance with repeated measures was used to analyze aMT6s excretion and onset times. Results showed that extraocular light did not suppress aMT6s excretion during the light therapy and no phase delay was detected on the following night after the light exposure. On the contrary, it was observed that extraocular light increased aMT6s excretion during light therapy and there was a compensational decrease during the offset time. The result was confirmed by another experiment which showed that aMT6s excretion was not suppressed (but increased) even after an 8-h extraocular light exposure. Both aMT6s levels and aMT6s concentrations normalized with creatinine were found to be reliable indices for melatonin production. Also, it was shown that single morning urine specimens could replace different time point specimens to determine the overnight melatonin production. In conclusion, extraocular light enhances rather suppresses melatonin and it does not phase delay the aMT6s onset.