Sometimes I can offer empirical proof ... pageview counts that definitely drop during a certain time that coincides with game time. Last summer the golf duel between Tiger and Rocco, for example. With some particularly exciting day games, you can see a bit of the same phenomena with the NCAA tournament.
But it's not just sudden bites out of the traffic patterns. It's also a general traffic malaise as people opt to check brackets and basketball predictions rather than macro economic forecasts and yield curve debates.
Some research outfits take the exercise further, multiplying average salaries by projected time distracted by the games to come up with a dollar figure for lost productivity. I don't buy into such math, fun as it is. Everybody has goof-off time built in at work. The question is whether you spend it gossiping, checking out market plays, or checking out college basketball scores.
At the end of the day, sports sites are just as much competition for me as my regular business news rivals. At least during certain times of the year.