Based on epidemiological studies, Ian Morgan, a myopia researcher at the Australian National University in Canberra, estimates that children need to spend around three hours per day under light levels of at least 10,000 lux to be protected against myopia. This is about the level experienced by someone under a shady tree, wearing sunglasses, on a bright summer day. (An overcast day can provide less than 10,000 lux and a well-lit office or classroom is usually no more than 500 lux.) Three or more hours of daily outdoor time is already the norm for children in Morgan's native Australia, where only around 30% of 17-year-olds are myopic. But in many parts of the world — including the United States, Europe and East Asia — children are often outside for only one or two hours.