From Scientific American, a remarkable article about why television is addictive—it grabs your attention by taking advantage of your biological response to novel stimuli, then lulls you into relaxation. From the article:
What is more surprising is that the sense of relaxation ends when the set is turned off, but the feelings of passivity and lowered alertness continue. Survey participants commonly reflect that television has somehow absorbed or sucked out their energy, leaving them depleted. They say they have more difficulty concentrating after viewing than before. In contrast, they rarely indicate such difficulty after reading. After playing sports or engaging in hobbies, people report improvements in mood. After watching TV, people’s moods are about the same or worse than before.