Purpose: To objectively assess daily light exposure and physical activity levels in myopic and emmetropic children.
Methods: One hundred two children (41 myopes and 61 emmetropes) aged 10 to 15 years old had simultaneous objective measures of ambient light exposure and physical activity collected over a 2-week period during school term, using a wrist-worn actigraphy device (Actiwatch 2). Measures of visible light illuminance and physical activity were captured every 30 seconds, 24 hours a day over this period. Mean hourly light exposure and physical activity for weekdays and weekends were examined. To ensure that seasonal variations did not confound comparisons, the light and activity data of the 41 myopes was compared with 41 age- and gender-matched emmetropes who wore the Actiwatch over the same 2-week period.
Results: Mean light exposure and physical activity for all 101 children with valid data exhibited significant changes with time of day and day of the week (p < 0.0001). On average, greater daily light exposure occurred on weekends compared to weekdays (p < 0.05), and greater physical activity occurred on weekdays compared to weekends (p < 0.01). Myopic children (n = 41, mean daily light exposure 915 ± 519 lx) exhibited significantly lower average light exposure compared to 41 age- and gender-matched emmetropic children (1272 ± 625 lx, p < 0.01). The amount of daily time spent in bright light conditions (>1000 lx) was also significantly greater in emmetropes (127 ± 51 minutes) compared to myopes (91 ± 44 minutes, p < 0.001). No significant differences were found between the average daily physical activity levels of myopes and emmetropes (p > 0.05).
Conclusions: Myopic children exhibit significantly lower daily light exposure, but no significant difference in physical activity compared to emmetropic children. This suggests the important factor involved in documented associations between myopia and outdoor activity is likely exposure to bright outdoor light rather than greater physical activity.