A study by grad students at MIT produced this chart describing the average activity on 4chan over a two-week span. Those familiar with the site will not be surprised that it really livens up in the off-hours.
You'll notice the highest volume of posts occur in the midnight hours, while workday hours are relatively quiet. (Due to the rapid turnover of posts, they disappear more quickly when there's a higher volume.) This means either most 4channers have 9-5 jobs or, as was suggested on Twitter, 4channers sleep all day, rising only as the sun goes down to haunt the Internet and troll dead teen's Facebook pages. Vampires!
Compare the chart above to traffic patterns on Reddit and Digg, which hew more-or-less to the work day (left). How much of 4chan's repulsive culture is due to the fact that it's just a bunch of people up really late on a weekday night with nothing better to do than fuck around on the internet? We're not MIT grad students, but we'll say 40 percent. [MIT (PDF), Reddit and Digg data via Raterush]