Research that will be published in the Journal of Clinical Sleep Medicine finds workers with windowless office sleep less than those with windows, Fast Company reports.

Overall, the findings suggest that the health benefits of exposure to daylight during the work day extend far beyond quitting time and even beyond the work week. In addition to more overall light exposure, these workers sleep better, seem more active, and have higher quality-of-life ratings than those who work in artificial light all day. The source of the sleep troubles, in particular, might be disruption of their circadian rhythms–the internal clocks that operate best when exposed to sufficient daylight.

“Some say we spend 90% of our lives indoors,” says Boubekri, who’s also written a book on the role of lighting in architectural design. “It’s very, very significant.”