EFFECTS OF INDIRECT LIGHT AND PROPRANOLOL ON MELATONIN LEVELS IN NORMAL HUMAN-SUBJECTS

An indirect lighting protocol was developed to measure nocturnal melat onin suppression by light in normal human subjects. Goals were to mini mize both discomfort due to staring intensely at a bright light source , and behavioral variation due to wandering gaze. Subjects sat with a bank of five full-spectrum light sources placed behind them. Lights re flecting off the surfaces before each subject produced a hemisphere of light that measured 500 Ix +/- 5%. Subjects retired to bed in darknes s by midnight and then sat in the hemisphere of light from 02.00 h to 04.00 h. Blood for melatonin was drawn at 20-30-min intervals from mid night to 06.00 h. Plasma melatonin was measured by radioimmunoassay. T he indirect lighting protocol was used to compare the effects of 500 I x light to dark (21 subjects) and to study varying light intensities f rom 300 to 2000 Ix (7 subjects). We studied the effects of the sitting posture in very dim light of 20-30 Ix (6 subjects). We also studied t he effects of propranolol plus dark and propranolol plus 500 Ix light on melatonin levels. Subjects received placebo, 10 mg propranolol or 4 0 mg propranolol orally at 23.00 h, and were then exposed to either th e dark or light condition. Melatonin levels obtained with the indirect lighting protocol were consistent with studies using direct lighting; light of 500 Ix significantly suppressed nocturnal melatonin and supp ression was dose related between 300 and 2000 hi. Sitting in dim light had no significant effect on melatonin suppression when compared with the supine posture in the dark in six subjects. Propranolol caused a dose-dependent decrease in melatonin levels in both the dark and the l ight. There was no relationship between suppression of melatonin by pr opranolol and suppression by light. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.