I've read conflicting accounts around this. Some link "time outdoors" to exposure to "natural" light, which 99% of the time is brighter light. But others link time outdoors to a wider variety of focal distances. Someone walking in the woods is constantly refocusing from close to far, whereas someone sitting indoors is probably just staring at a fixed distance (computers/books etc). So the answer may be brighter indoor lighting (brighter computer screens) or maybe we need to continuously change our focal distances, something far more complicated to implement indoors.