The Test Re-Test Reliability of the Melatonin Suppression by White Light

Abstract

The melatonin supersensitivity to light has been suggested as a biological marker of bipolar disorder. However previous studies have been inconsistent with regard to light induced suppression of melatonin and raising questions regarding its reproducibility and reliability. The present study examined the test re-test reliability of the melatonin suppression by light in healthy subjects. Study was divided into two parts. The first examined the melatonin suppression by 200 lux of light while the second examined effect 500 lux of light. Subjects were tested twice, separated by one week for each part of the study. On each night subjects reported to the study at 1800 h. The first sample was collected at 2100 h (in the light). Subjects were then placed in a dark room, with a background light intensity of 10–20 lux. Further blood samples were collected at regular intervals. After each collection, blood samples were centrifuged and plasma separated and stored frozen at –20ºC. Plasma melatonin concentrations were determined by a specific radioimmunoassay. Results showed poor test re-test reliability for nights 1 and 2 for both light intensities suggesting that the melatonin suppression by light is not reproducible and has poor reliability. The poor test re-test reliability may provide an explanation for the inconsistencies in previous studies.