Prior studies show that knowledge work is characterized by highly interlinked practices, including task, file and window management. However, existing personal information management tools primarily focus on a limited subset of knowledge work, forcing users to perform additional manual configuration work to integrate the different tools they use. In order to understand tool usage, we review literature on how users' activities are created and evolve over time as part of knowledge worker practices. From this we derive the activity life cycle, a conceptual framework describing the different states and transitions of an activity. The life cycle is used to inform the design of Laevo, a temporal activity-centric desktop interface for personal knowledge work. Laevo allows users to structure work within dedicated workspaces, managed on a timeline. Through a centralized notification system which doubles as a to-do list, incoming interruptions can be handled. Our field study indicates how highlighting the temporal nature of activities results in lightweight scalable activity management, while making users more aware about their ongoing and planned work.