Traditional flow production is experiencing a dynamic change towards smart factories. In the context of Industry 4.0 new technologies for worker assistance can be implemented in assembly, logistics or maintenance. Using Augmented Reality (AR) in order picking processes can provide advantages like time and error reduction and subsequently cost decrease. But what is the impact on employees wearing smart glasses during a full shift? Conducting a field study on the shop floor of an automotive production plant with expert order pickers, we analyze health-oriented aspects as well as the task completion time and the error frequency for a full shift usage of smart glasses in order picking processes. We face challenges such as an interaction with the warehouse management system and a predetermined production rhythm to ensure reliable results for the 8-hour usage of smart glasses, which may point the way towards future digital manufacturing.