The prevalence of smartphone use among young developing brains makes it necessary for us to determine how excessive smartphone use is shaping both neurological and cognitive development. Consider, for example, that grey matter is critical to learning and reasoning, and that smartphone dependence has been linked to decreased grey matter volume. Smartphones are known to affect cognition (including classroom learning), but little is known about how precisely how they impact the dynamics of the learning process during a lecture. This study examined the point at which smartphone use (or exposure) was most likely to distract students and impair learning of lecture material (as measured by low-stakes test performance). Almost 400 college students were presented with a lecture while researchers manipulated the subjects’ phone access. Afterward, they took a test on the lecture content and completed a questionnaire about the fear and discomfort that they experienced when their phone access was restricted; a concept known as nomophobia. This study revealed that students who had access to their phones performed worse on the material that was presented in the third quarter of the lecture. Those who experienced more nomophobia performed worse on the memory and especially on the material in the third quarter of the lecture. This indicated that, in short lectures, smartphones have their largest impact on attention and learning at 10 – 15 minutes into the class.