Do you feel held captive by your phone? All those rings, pings, buzzes and haptics are triggering the involuntary part of your brain and cause you to lose focus and work harder to concentrate.
When you drive in town and stop at a light, you aren’t immediately back up to 35 mph when you start again. You have to speed back up. Same with your brain. It takes time to get back to that level of focus.
Turn off your email and social media notifications. Take back control of your attention, and do not let a device steal your time.