A study involving 49 office workers, of whom 27 worked in windowless workplaces and 22 in workplaces with windows, has found that those exposed to natural light slept better, and were also more active.

Employees with windows in the workplace received 173% more white light exposure during work hours and slept an average of 46 minutes more each night.

Workers without windows had lower scores on quality of life measures related to physical problems and vitality, as well as poorer outcomes on measures of overall sleep quality and sleep disturbances.

http://www.futurity.org/windows-workers-sleep-health-745022/

Boubekri M, Cheung IN, Reid KJ, Wang CH, Zee PC. Impact of windows and daylight exposure on overall health and sleep quality of office workers: a case-control pilot study. J Clin Sleep Med 2014;10(6):603-611.