The authors of the studies in California were not doctors, but architectural researchers who study buildings.
The research was conducted by an energy consulting firm for the California state Board for Energy Efficiency and Pacific Gas and Electric Co. The state utility's goal was to save on energy costs and determine whether sunlight increases human productivity. They were surprised at how strong the correlation was between daylight and productivity.
First, they examined student test scores and classrooms at three school districts in California, Colorado, and Washington state. They found that students in classrooms with the most daylight did 20 percent better on math tests and 26 percent better on reading tests than students at the same school in classrooms with the least amount of natural light.
In a companion study, the researchers looked at 108 stores that were part of a large chain. The stores were virtually identical in layout, except that two-thirds of the stores had skylights. They then looked at the sales figures for the various stores and determined that a skylight system increased sales by 40 percent.
The reports are the first large studies of this kind, and were not done by doctors or peer-reviewed by other experts. But the researchers were careful to control other factors such as how affluent a community was or where stores and schools were located. They found that the association between sunlight and productivity were strong regardless of location.
It is not known why daylight might affect human productivity, but one possibility is that light has an effect on melatonin, a brain chemical that helps regulate sleepiness and alertness. Sunlight suppresses melatonin, making us more alert and full of energy. Conversely, too much of the chemical can make people drowsy or depressed.
Doctors suspect that melatonin plays a part in the "winter blues," when people feel more sleepy and depressed during the winter season. Some doctors use light therapy on patients who are depressed, having them sit in front of strong lights for a few hours each day.