Light-induced melatonin suppression in school-aged children before and during camping: associations with circadian phase and daytime light exposure

Prior light exposure can influence the magnitude of melatonin suppression in response to evening light, even within the same individual. We examined whether light-induced melatonin suppression in school-aged children differed before and during a camp with approximately two weeks of exposure to a natural light-dark cycle. We also explored interindividual variation in melatonin suppression and its associations with circadian phase, habitual sleep timing, and daytime light exposure. Twenty-one healthy children aged 9--14 years underwent evening light experiments at the start of camp (Pre-camp session) and approximately two weeks later (Camp session). Melatonin suppression was calculated by comparing the area under the melatonin concentration curve between the evening light condition (200 {+/-} 30 lx, 5000 K) and the dim light condition (< 30 lx, incandescent lamps), with integration starting at dim light melatonin onset (DLMO). We assessed sleep habits from 14-day sleep diaries before camping and measured wrist-level illuminance during camping. We examined associations using Spearman correlations and compared Pre-camp and Camp melatonin suppression using a paired t-test. Melatonin suppression did not differ significantly between Pre-camp and Camp sessions (p = 0.21). Greater suppression in the Pre-camp session was associated with later DLMO (rho = 0.49, p = 0.028), but not with habitual sleep variables. In the Camp session, higher same-day geometric mean illuminance was associated with lower suppression (n = 15; rho = -0.58, p = 0.024), whereas 7-day light-exposure metrics were not. Mean melatonin suppression did not significantly change across the camping period. Meanwhile, children showed large interindividual variation in melatonin suppression, and circadian phase and recent daytime light exposure are candidate correlates of this variation in the responsiveness to the evening light.