Creator Bård Edlund explains:
The further a circle is to the left, the more certain it is that most citizens in the country it represents are awake. The opposite is true for the right/night side. If you're wondering when to tweet your most brilliant thoughts in order to maximize International exposure, it looks like 9am EST or thereabouts is a pretty good bet!
The above graphic is only a screenshot. Click here to see the circles in motion.
The Dish 