Daily outdoor exposure in children aged 7–9 years in Shanghai averages 90 minutes with a mean sunlight intensity of 2345 lux, which is lower than levels reported in other countries and insufficient to fully offset myopia risk.

When children are exposed to bright sunlight for at least 15 minutes straight, special cells in the eye release a chemical called dopamine, which tells the back of the eye to stop growing too long. At the same time, the blood vessels in the layer behind the retina widen and carry more oxygen, which also stops the eye from stretching out. If the sunlight is not bright enough or the time outside is broken into short chunks, this process does not turn on fully, and the eye keeps growing longer, leading to nearsightedness.