Suppression of melatonin by 2000-lux light in humans with closed eyelids

Background: In order to clarify the role of light in regulating body functi ons in sleeping humans, we studied whether the light-sensitive pineal hormo ne melatonin can be suppressed by facial light exposure in subjects with cl osed eyelids. Methods: Eight healthy volunteers participated in 3 nightly sessions: a dim -light control session (<10 lux) and two light-exposure sessions (2000 lux, 60 min between 2400 and 0200 h). One light exposure occurred with eyes ope n and the other with eyes closed. Saliva samples were collected at least ev ery hour from 1900 to 0300 h, Melatonin concentrations were measured by rad ioimmunoassay. Results: Salivary melatonin concentrations decreased only in 2 of the 8 vol unteers during light-exposure sessions with eyes closed. On average, light exposure did not decrease the salivary melatonin concentration. Conclusions: Because indoor illuminance is usually much lower than 2000 lux , light is probably ineffective in regulating the neuroendocrine hypothalam ic functions in people during their sleep. Nevertheless, the possibility re mains that higher illuminances, often used for therapeutic purposes, can in hibit the secretion of melatonin even in sleeping patients. (C) 1999 Societ y of Biological Psychiatry.