Influence of seasons upon personal light exposure and longitudinal axial length changes in young adults

Purpose To investigate the association between objectively measured ambient light exposure and longitudinal axial length changes (and their seasonal variations) over a period of 12 months in young adults. Methods This prospective longitudinal observational study included 43 healthy young adult university students (21 emmetropes and 22 myopes) aged between 18 and 30 years. Three axial length measurements were collected at 6-month intervals (i.e. at baseline, 6 and 12 months), in summer and winter to determine the axial eye growth. Personal ambient light exposure data were measured in winter and summer months with wearable sensors, from which the mean daily time exposed to bright (outdoor) light levels (>1000 lux) was derived. Results Greater daily bright light exposure was associated with less axial eye growth (beta = -0.002, p = 0.006) over 12 months. In summer, myopes exhibited significantly greater changes in axial length (mean change 0.04 +/- 0.05 mm) compared to emmetropes (-0.01 +/- 0.05 mm) (p = 0.001), but there was no significant difference between refractive groups in winter. Emmetropes also spent significantly greater time in outdoor light levels in summer compared to winter (p < 0.0001), while myopes spent similar time outdoors during both seasons (p = 0.12). Differences in light exposure between summer and winter were also associated with seasonal differences in axial eye growth (p = 0.026). Conclusion In young adults, greater time spent in bright light was associated with slower longitudinal axial eye growth. Seasonal light exposure and axial length changes were dependent on refractive error in this population and also exhibited an inverse relationship.