Research has explored head-worn displays (HWD) in various professional contexts. However, evaluations have been limited by short-term use, a focus on the person using the HWD, and on performance variables. In a field study, we evaluated a monocular, opaque HWD for multi-patient monitoring, which supervising anesthesiologists wore for 8-10 days each. We investigated the effect of prolonged HWD use on the experience of the supervising anesthesiologists and their co-workers using interviews and repeated observations. A reflexive thematic analysis showed (1) interaction and mindset changes over time, (2) information on the HWD is more than numbers, (3) the HWD affects co-workers' collaboration with supervisors, and (4) distraction depends on the point of view. Using activity theory, we discuss the fact that HWD use develops and changes over time and that even a single-user HWD influences the collaboration with co-workers. We conclude with implications for HWD design, implementation, and evaluation.