Reddit reviews The Plug-In Drug: Television, Computers, and Family Life

This book will answer your question.

TLDR:

• kids (and adults) get addicted, moody, hyperactive, go through withdrawal without TV
  
  
• attention span and ability to delay gratification are diminished
  
  
• watching TV the brain goes into a "passive" state that doesn't occur while reading, this state is not actually relaxing (actually low-level stress)...
  
  That's off the top of my head, there's definitely more.