Intensive outdoor time increased choroidal thickness in school children in Japan in just one week, a study found.  

Examining the effect of short-term intense outdoor activities (average of 6.15 ± 2.98h per day) for a week on 24 schoolchildren (11.5 ± 0.5 years, 50% female) researchers found that choroidal thicknesses increased significantly (30.7 ± 20.3μm (p < 0.001)), while refractive errors, axial length and retinal thickness didn’t differ significantly.  

This is an important phenomenon in understanding the mechanism by which outdoor activity inhibits myopia progression, authors noted. “Further studies are clearly needed to determine the extent to which the increased choroidal thickness can be maintained over a short period of time and whether other factors contribute to these phenotypes. Taken together, our results may provide a new strategy to control choroidal thickness through outdoor activities.”