These graphs were done by Twitter engineers - Miguel Rios and Jimmy Lin - and they found some interesting things:
We see different patterns of activity between the four cities. For example, waking/sleeping times are relatively constant throughout the year in Tokyo, but the other cities exhibit seasonal variations. We see that Japanese users' activities are concentrated in the evening, whereas in the other cities there is more usage during the day. In Istanbul, nights get shorter during August; Sao Paulo shows a time interval during the afternoon when Tweet volume goes down, and also longer nights during the entire year compared to the other three cities.It's also interesting to note that one can see a marked effect in NYC of what appears to possibly be the start of the academic year (see the shift in the start of September) in addition to the seasonality that is visible there.
I wonder if the relatively low number of Tweets in Tokyo during the day have to do with the medium of use. For example, do people make more cellphone texts from 10am to 6pm than they do from 7pm to 2 am? After all, there is no reason to believe that Twitter has a major corner in the short-texts market in Tokyo (where Facebook is also not a major player).
I also noticed the lags in mid-afternoon Tweets in Sao Paulo, and I wonder if that's got something to do with long lunches that they take down there. It would be interesting to see if how comparable it is with other Latin American cities - where far-longer-than-American lunches are commonplace.