Outdoor light exposure during childhood is the most critical known modifiable risk factor for myopia. Outdoor light exposure prevents or delays the onset of myopia and may slow progression. (Ho et al., 2019) The protective effect of being outside is currently explained by high light intensity triggering the release of retinal dopamine, an ocular growth inhibitor that inhibits myopic development. (Ramamurthy et al, 2015) Another potential explanation is that pupils are more constricted outdoors, causing a greater depth of field and less image blur, resulting in less myopic stimulus. (Flitcroft 2012)
Most well-controlled studies show a significant effect of outdoor light exposure when children have a minimum of 2 hours of intermittent outdoor light exposure per day, including recess time, physical education time and additional outdoor time after school. (Wu et al., 2020, Ho et al., 2019, Lan et al, 2014) Research also shows that children must avoid working distances of less than 12 inches (30 cm). (Huang et al, 2019)
I strongly recommend the 20-20-2 Rule to all children, especially those at risk of developing myopia and those already myopic.
