Harvard Business Review (paywall):

In recent research, we investigated whether merely having one’s own smartphone nearby could influence cognitive abilities. In two lab experiments, nearly 800 people completed tasks designed to measure their cognitive capacity. In one task, participants simultaneously completed math problems and memorized random letters. This tests how well they can keep track of task-relevant information while engaging in a complex cognitive task. In the second task, participants saw a set of images that formed an incomplete pattern, and chose the image that best completed the pattern. This task measures “fluid intelligence,” or people’s ability to reason and solve novel problems. Performance on both of these tasks is affected by individuals’ available mental resources.

Our intervention was simple: before completing these tasks, we asked participants to either place their phones in front of them (face-down on their desks), keep them in their pockets or bags, or leave them in another room. Importantly, all phones had sound alerts and vibration turned off, so the participants couldn’t be interrupted by notifications.

The results were striking: individuals who completed these tasks while their phones were in another room performed the best, followed by those who left their phones in their pockets. In last place were those whose phones were on their desks. We saw similar results when participants’ phones were turned off: people performed worst when their phones were nearby, and best when they were away in a separate room. Thus, merely having their smartphones out on the desk led to a small but statistically significant impairment of individuals’ cognitive capacity — on par with effects of lacking sleep.