“This includes, crucially, checking e-mail, as well as browsing work-related websites. In both cases, even a brief intrusion of work can generate a self-reinforcing stream of distraction that impedes the shutdown advantages described earlier for a long time to follow (most people are familiar, for example, with the experience of glancing at an alarming e-mail on a Saturday morning and then having its implications haunt your thoughts for the rest of the weekend).”