30th Apr 2024

Light has an impact on human sleep and circadian rhythm. Morning sunlight has an important role in adjusting the human circadian rhythm to 24 hours, but exposure to artificial light at night can be a cause of the delay of the circadian rhythm and sleep timing. Our previous studies showed that the effects of light at night are greater in children than adults. In the present study, we examined if the individual difference in sleep timing and circadian phase in children is related to the magnitude of melatonin suppression by light at night, an index of circadian sensitivity to light at night. Twenty-one healthy children aged 9 to 14 participated in this experiment. Informed consent was obtained from all participants and their parents. The participants were asked to record a sleep diary two weeks before the experiment. Saliva samples were collected to measure melatonin concentrations every hour from 17:00 to the habitual bedtime under ordinary room light conditions (200 ± 30 lx) on the first day and under dim light conditions (<30 lx) on the second day. Dim light melatonin Onset (DLMO) was used as an index of the circadian phase, and the percentage of melatonin suppression by light was calculated as an index of circadian sensitivity to light. Individual difference in the percentage of melatonin suppression was correlated with DLMO (r=0.535, p=0.039) and habitual sleep onset time (r=0.503, p=0.056). The children sensitive to light showed delayed circadian phase and sleep onset time. This result suggests that individual differences in circadian sensitivity to light before bedtime influence circadian rhythm and sleep in children.