With our heavy use of digital media, it could be said that we have taken multitasking to new heights, but we’re not actually multitasking; rather, we are switching rapidly between different activities. Adrenaline and cortisol are designed to support us through bursts of intense activity, but in the long term cortisol can knock out the feel-good hormones serotonin and dopamine in the brain, which help us feel calm and happy, affecting our sleep and heart rate and making us feel jittery.
Source: The lost art of concentration: being distracted in a digital world | Life and style | The Guardian