Abstract. Study of mood and in turn mood changes is an important index of general wellbeing state and can also be an indicator of various mood disorders including clinical depression and bipolar disorder. While there have been clinical studies of mood, less emphasis has been placed on the factors that affect the mood, especially in workplace. The typical approach taken by these studies is to use clinical questionnaires in order to record the activities that impact the mood. However, recording activities that may have an influence on the mood in this manner suffers from a number of issues including floor effect and difficulty in recalling past activities. Our work instead has focused on using unobtrusive monitoring technology to study mood changes during office hours and a set of associated factors that influence these changes. We have primarily studied two factors and their impact on the mood changes, namely social activity and nonsedentary work style. The pilot study ran over the course of 7 days of measurements with the participation of 9 knowledge workers. The results have shown that mood changes are highly correlated with both social interactions and non-sedentary work style. This study is the first to investigate the correlation between mood changes and non-sedentary behavior patterns, opening up a research avenue to explore psychological effects of increasing prevalence of sedentary behavior.