Lighting design for office buildings has focused largely on providing sufficient light for visual performance, minimal glare, good colour rendering and energy conservation. Little attention has been given to understanding how light affects the non-visual systems, including circadian regulation that affects sleep and mood. Circadian light-dark and activity-rest patterns of individuals working in a building designed to provide daylight availability in the space were obtained in winter and summer. Measures of objective and subjective sleep and self-reports of mood were also obtained from participants. Results show a significant increase in light exposure during summer. Sleep quantity and quality were also significantly higher in summer than in winter. Strategies to increase circadian light exposures in buildings should be a consideration in architectural design.