Recent epidemiological evidence suggests that children who spend more time outdoors are less likely to
be, or to become myopic, irrespective of how much near work they do, or whether their parents are
myopic. It is currently uncertain if time outdoors also blocks progression of myopia. It has been suggested
that the mechanism of the protective effect of time outdoors involves light-stimulated release of
dopamine from the retina, since increased dopamine release appears to inhibit increased axial elongation,
which is the structural basis of myopia. This hypothesis has been supported by animal experiments
which have replicated the protective effects of bright light against the development of myopia under
laboratory conditions, and have shown that the effect is, at least in part, mediated by dopamine, since the
D2-dopamine antagonist spiperone reduces the protective effect. There are some inconsistencies in the
evidence, most notably the limited inhibition by bright light under laboratory conditions of lens-induced
myopia in monkeys, but other proposed mechanisms possibly associated with time outdoors such as
relaxed accommodation, more uniform dioptric space, increased pupil constriction, exposure to UV light,
changes in the spectral composition of visible light, or increased physical activity have little epidemiological
or experimental support. Irrespective of the mechanisms involved, clinical trials are now underway
to reduce the development of myopia in children by increasing the amount of time they spend
outdoors. These trials would benefit from more precise definition of thresholds for protection in terms of
intensity and duration of light exposures. These can be investigated in animal experiments in appropriate
models, and can also be determined in epidemiological studies, although mo