Head-worn displays have the potential to assist professionals in a wide variety of contexts by providing heads-up and hands-off information in real-time. For instance, nurses may be able to use head-worn displays to maintain peripheral awareness of the well-being of multiple patients while away from the nurses' station and the patients' beds. However, little is known about the advantages and disadvantages of head-worn displays in such contexts. In fact, many studies have shown that head-worn displays can lead to detriments in attention and visual performance. In three experiments, we tested people's ability to detect simple, abrupt onset stimuli in peripheral vision when using a traditional computer monitor or Google Glass™. When using Google Glass, participants were significantly less likely to detect peripheral events. These data indicate that monocular optical see-through head-worn displays can make it harder to see peripheral stimuli. Stimuli need to be developed for head-worn displays that preserve their benefits for mobile users, while overcoming some of their disadvantages.