Light and darkness fail to regulate melatonin release in critically ill humans.

Abstract

Pineal dysfunction has been associated with morbidity and mortality in various animal models of severe illness, and low melatonin plasma concentrations have recently been reported in patients on the ICU. However, it is not known whether the physiological response of the pineal gland to light and darkness is preserved in critical illness. We examined the nocturnal release of melatonin in response to 1 h of darkness followed by 1 h of bright light (>10,000 lux) in 20 severely ill patients on a medical ICU. Eleven elderly (median age 73 years) and 9 young patients (38 years) were recruited. Melatonin plasma concentrations were measured at 30-min intervals. In 15 patients melatonin plasma concentration was low and responded to neither darkness nor light. In three elderly and two young patients melatonin plasma concentrations were elevated irrespective of illumination. The physiological regulation of melatonin secretion by darkness and light is abolished in severely ill patients.