Annual Solar Cycles: Investigating Seasonal Variations in Southern Norway and Their Potential Role in Human Eye Growth.

PURPOSE: Human eye growth is reported to follow a seasonal pattern, with accelerated eye growth leading to myopia being linked to limited exposure to daylight. To investigate how environmental lighting may contribute to eye growth and refractive errors in emmetropic children, we analyzed real-world seasonal variations in daylight intensity, duration, and spectrum and their association with healthy regulated human eye growth. METHODS: Downwelling irradiance was collected around the 2 solstice and the 2 equinox dates to assess variations across 4 seasons at 60°N latitude location over 4 years (2022-2025). To assess seasonal changes in the relative levels of short- versus long-wavelength parts of daylight, we calculated ratios of alpha-opic equivalent daylight illuminance (EDI) for the photoreceptors. Gold-standard ocular measurements were obtained from emmetropic 7 to 11-year-old children who spend 1 to 2 hours outdoors every day and were used to explore the associations between healthy eye growth rates and seasonal variations in these ratios. RESULTS: Daylight intensities were significantly lower during autumn and winter mornings, winter midday, as well as autumn and spring afternoons. The spectra were significantly short-wavelength enriched during winter mornings and midday, and spring afternoons. The seasonal cycle of daylight and human eye growth in 7 to 11-year-old children was in anti-phase, with peak growth delayed relative to the shortest winter days when daylight was short-wavelength enriched. CONCLUSIONS: The short-wavelength enrichment encompasses the sensitivities of both S opsin and melanopsin and invites speculation that the eye may have evolved to exploit seasonal variations in intensity and spectrum as cues that may contribute to the regulation of eye growth via photoreceptor opsin activation.