Is the question about what time of the day USCIS send emails?
I did not get an approval mail yet, but other status change emails were sent exactly at 12 noon EDT. I have seen people got emails at 7:00 am (I am guessing PDT), which is 10am EDT. So, I think they send in batches, depending on your receipt no, or A#, or something else.
Also, the emails are sent one day after the actual status change, or approval. The web page is central for all service centers and they may move the data from all service centers overnight or early in the morning.
-NS