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Cognitive capacity is the total amount of information the brain is capable of retaining at any particular moment.
Your cognitive capacity is slightly reduced when your smartphone is very close to you.
Researchers measured cognitive capacity of different smartphone users and asked them to keep their phones before participating in the assessment.
They discovered that participants with their phones in another room significantly perform better than those with their phones on the desk, and they also slightly performed better than those participants who kept their phones in a pocket or bag.
Having a smartphone very close to you reduces your ability to focus and perform tasks at hand very well because part of your brain is actively connected to the phone whether you receive notifications at that moment or not.