This study examines the effect of daylighting on human performance, focusing on skylighting as a way to isolate daylight as an illumination source, and separate illumination effects from other qualities associated with daylighting from windows. It establishes a statistical connection between daylighting and student performance and between skylighting and retail sales. Using multivariate linear regression analysis, the study examined 21,000 school records from 3 school districts in 3 states and daylighting conditions in over 2,000 classrooms. Data indicate students with the most classroom daylighting progressed 20 percent faster on math tests and 26 percent on reading tests in one year than those with the least. Similarly, students with the largest windows progressed 15 percent faster in math and 23 percent faster in reading than those with the least. In classrooms where windows could be opened, there was a 7 to 18 percent faster educational progress than those with fixed windows, regardless of air conditioning. These findings are reported to be consistent regardless of curricula or teaching styles. Appendices provide data charts, a report from one of the classroom-level analyses, sample illumination readings, classroom plans and sections, and photographs of schools and classrooms. (GR)