0063 UNEXPECTED INCREASE IN MELATONIN CONCENTRATIONS DURING DAYTIME SLEEP IN SIMULATED NIGHT WORK PROTOCOL

Abstract

Melatonin is known to be regulated by two main processes: circadian control and acute light suppression. Models based on this understanding predict that under continuous dim light conditions the circadian control causes melatonin to be high during the biological night and low during the biological day, and that light at night suppresses melatonin production. However, these models have been built using group-averaged data and may not reflect individual differences. In studying 24-h melatonin profiles collected during a highly-controlled simulated night work protocol, we were struck by the observation that melatonin concentrations in some individuals are significantly increased during daytime sleep and substantial interindividual differences exist in the degree of light-induced melatonin suppression. Methods: Fourteen healthy young adults (mean age ± SD, 28 ± 9 y) underwent two 8-day laboratory protocols: a day shift protocol and a simulated night shift protocol (12-h inverted behavioral and environmental cycles for three shifts). After four adaptation days, light levels were ~90 lux (moderate intensity “room” light) during scheduled wakefulness and 0 lux during scheduled sleep in the last three shifts of both protocols. Hourly melatonin concentrations were assessed for 24 h on the first and third day and night shift. Results: 11 out of 14 participants had a significant increase in melatonin levels during daytime sleep as compared to that during the same clock time under the day shift, with 2.63 ± 0.27 fold change (mean ± SEM). Moreover, there were large inter-individual differences in the magnitudes of light-induced melatonin suppression during the biological night under simulated night work protocol, ranging from 31% to 98% of suppression. Conclusion: Our observations raise the question whether sleep can stimulate melatonin production during the biological day. New models of melatonin regulation should take into consideration the possible effect of sleep and large individual differences in acute light suppression.