Philip Zimbardo

Every time you log in to Facebook, every time you click on your News Feed, every time you Like a photo, every time you send anything via Messenger, you add another data point to the galaxy they already have regarding you and your behavior.

Even though we don't always realize it, as the day goes on, we have increased difficulty exerting self-control and focusing on our work. As self-control wears out, we feel tired and find tasks to be more difficult, and our mood sours.