A little chime sounds, indicating that you’ve gotten a text message, and it activates the stress hormones in your head. Your body goes into fight-or-flight mode and your muscles may even contract, preparing you to run. Now imagine that process being repeated hundreds of times, perhaps thousands, over the course of a day, every time your phone vibrates or makes noise. The effect can be so strong that you may feel sore after your muscles spent the day contracting and releasing.
