The presence of mobile devices (e.g., smartphones, tablets and computers) in the classroom gives students the possibility of doing off-task activities during lectures. The purpose of this mixed-method field study was to learn more about students’ behaviors, reasons, and opinions regarding such activities and their consequences on learning. This study is one of few to take a holistic view on this topic by taking the use of all technical devices in class into account and assessing its consequences on learning objectively. This is important to gain a full picture concerning the consequences of off-task activities in class. Right after a lecture, bachelor students (N = 125) answered a survey containing questions on their usage of mobile devices during this last class. Furthermore, they took a test on the content of that lecture. Qualitative and quantitative analysis of data revealed that students spent an average of more than 19% of their time using a digital device for non-class purposes. Interestingly, this was not significantly linked with learning, although many students reported being aware of this behavior’s potential negative consequences. But there was a significant negative link between the number of received notifications and learning. These results suggest that external interruptions have a stronger negative effect than internal interruptions, allowing us to make better recommendations on how to use electronic devices in the classroom.