Sleep mode lets users go for days or weeks without rebooting, supporting work on multiple tasks that they can return to later. However, users also struggle with window clutter, facing an increasing number of 'left-over windows' that get in the way. Our goal is to understand how users create and cope with left-over windows. We conducted a two-week field study with ten notebook users. We found that they work in very short sessions, switching often between computer-based and external tasks. 34% of left-over windows remain untouched for a day or more, increasing in quantitity until they all disappear after a reboot. Some users reboot as a deliberate 'clean-up' strategy, whereas others lose left-over windows after an unexpected system crash. Users intentionally keep left-over windows as to-do lists, as reminders of upcoming tasks, and for facilitating future access; the rest are simply forgotten. Tools for visualizing and managing left-over windows should help users reduce window clutter, while maintaing the benefits of interruptible work sessions.