There are sweeps that start at about 5 pm ATL time. In the past, the sweep could take as long as 45 minutes, with higher flight numbers processed last. More recently, the sweeps seem to be completed much earlier, perhaps by 5:15 or so ATL (Eastern) time, which is 4:15 in the midwest (Central time). The confirmation emails are sent out many hours later and do not reflect at all when the upgrade was granted.