An interruption is a randomly occurring, discrete event that breaks the continuity of cognitive focus on a primary task and typically requires immediate attention and demands action. This article investigates the effect of the timing of an interruption and source on both demanding and non-demanding tasks. Time logs of daily work activities from 21 white collar workers were analyzed. The findings show that interruptions have a negative impact on worker performance when they occur at the middle or end of the current/primary task. In addition, results show that the majority of the interruptions were externally generated (78%) rather than internally generated. The more demanding the tasks that were interrupted, the greater the negative impact on the overall performance. Suggestions for reducing the impact of interruptions are also discussed.