From Harvard Business Review, the authors of the latter explained the effect of reducing cognition:
“This cognitive capacity is critical for helping us learn, reason, and develop creative ideas. In this way, even a small effect on cognitive capacity can have a big impact, considering the billions of smartphone owners who have their devices present at countless moments of their lives. This means that in these moments, the mere presence of our smartphones can adversely affect our ability to think and problem-solve — even when we aren’t using them. Even when we aren’t looking at them. Even when they are face-down. And even when they are powered off altogether.”
The interesting takeaway from these studies is that physically distancing yourself from your phone decreases its cognitive load on your brain. The same “brain drain” applies to any potential distractor even when they aren’t actively distracting us. Research in cognitive psychology tells us that we automatically pay attention to things that are habitually relevant to us, even when we are focused on something else.