I know for me I see an automatic spike when I tweet an item -- so every 30 minutes (lol!)
BUT the busiest times for me seem to be: 1-3 a.m., 6 a.m., 8-10 a.m., 12-1 p.m., 8-10 p.m. EST
It donned on me a couple of weeks ago that I need to be tweeting 24hrs a day for the various time zones around the world and that's definitely helped drive traffic.
And I look at daily stats -- it is just way too addicting not to! =)