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

Background: In order to clarify the role of light in regulating body functions in sleeping humans, we studied whether the light-sensitive pineal hormone melatonin can be suppressed by facial light exposure in subjects with closed 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 open and the other with eyes closed. Saliva samples were collected at least every hour from 1900 to 0300 h. Melatonin concentrations were measured by radioimmunoassay. Results: Salivary melatonin concentrations decreased only in 2 of the 8 volunteers 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 hypothalamic functions in people during their sleep. Nevertheless, the possibility remains that higher illuminances, often used for therapeutic purposes, can inhibit the secretion of melatonin even in sleeping patients.