From the New York Times, in an article on why and how active people burn so many calories than the inactive:
At meetings, [Dr. Levine] stands instead of sitting. Talking on the telephone, he paces around. In his office he has a treadmill in place of a desk. …
“My computer is stationed over the treadmill,” he said. “I work at 0.7 miles an hour.”
A stand-up desk might seem simpler, but he prefers the treadmill.
“Walking at work, first of all it’s addictive,” he said. “It’s terribly good fun. I actually feel happier, particularly in the afternoon. You might think you come home exhausted, but you don’t. You come home energized.”
For him, the treadmill has eliminated the afternoon slump, when a lot of people feel sleepy and crave candy bars or caffeine.