Dan Ariely made a list. Number four:
Checking email is addictive in the same way gambling is. You see, years back the famous psychologist B.F. Skinner discovered that rats would work much harder if the rewards were unpredictable (rather than a treat every 5 times they pressed a bar, one would come after 4, then 13, etc). This is the same as email, most of it is junk, but every so often, it’s fantastic: an email from the woman you’ve been chasing for instance. So we distract ourselves from work by constantly checking and checking and waiting to hit the email jackpot. And to be perfectly honest, I’ve checked my email at least 30 times since starting writing this article.