Background It has been proposed that introducing daylight saving measures could increase children’s physical activity, but there exists little research on this issue. This study therefore examined associations between time of sunset and activity levels, including using the bi-annual ‘changing of the clocks’ as a natural experiment.

Methods 23,190 children aged 5–16 years from 15 studies in nine countries were brought together in the International Children’s Accelerometry Database. All children provided objectively-measured physical activity counts per minute from Actigraph accelerometers. Date of accelerometer data collection was matched to time of sunset, and to weather characteristics including daily precipitation, humidity, wind speed and temperature.

Results Adjusting for child and weather covariates, we found that longer evening daylight was independently associated with higher daily physical activity. Consistent with a causal interpretation, the magnitude of these associations was largest in the late afternoon and early evening and these associations were also evident when comparing the same child just before and just after the clocks changed. These associations were, however, only consistently observed in the five mainland European, four English and two Australian samples (adjusted, pooled effect sizes 0.03–0.07 standard deviations per hour of additional evening daylight). In some settings there was some evidence of larger associations with daylight in boys. There was no evidence of interactions with weight status or maternal education, and inconsistent findings for interactions with age.

Conclusion In Europe and Australia, evening daylight seems to play a causal role in increasing children’s activity in a relatively equitable manner. This suggests that, by shifting the mean of the entire population, the introduction of additional daylight saving measures could yield important public health benefits.