Not quite sure about that. I've seen people report getting approval emails at 10 PM through 11 PM. I presume that the service centers have a batch job that runs at designated times, that takes all cases demarcated for approvals by IOs and generates the emails. Presumably, cases that get approval emails early in the morning, like 8 AM EST had been marked for approval the afternoon before (since TSC doesn't open at 8 AM EST...they're an hour behind).
So I'm guessing that if cases get marked in the late afternoon, they might be picked up by the batch job that sends mail late night