To evaluate the serum melatonin levels in critically ill pediatric patients and to test the effect of light on the melatonin’s circadian rhythm. Data on melatonin secretion in critically ill pediatric subjects are lacking.

We investigated the serum melatonin levels of 16 sedated and mechanically ventilated patients in a pediatric intensive care unit. Children (mean age, 5.1 ± 3.1 years) were randomly assigned to a dark-exposed or to a light-exposed group to evaluate the effects of light on serum melatonin concentrations. Blood samples for serum melatonin analysis were collected at 10 p.m., 1 a.m., 3 a.m., 5 a.m., 8 a.m., and 12 p.m.

The melatonin circadian rhythm was severely disrupted in critically ill children; light exposure lowered serum melatonin even in a context of highly altered circadian cycle; melatonin peaks were greater for healthy age-matched children.

The high melatonin levels in the critically ill children may be a response to counteract the elevated oxidative stress associated with serious diseases. Whether these elevated melatonin levels confer any beneficial effects in pediatric critically ill patients remains unknown.