From what I've been able to ascertain, from 1AM to 5AM (local time) some (not all) of the iHeart station streams run either:
as mentioned by gr8oldies, morning show reruns (I've heard the Elvis Duran show on some streams) or
a national, generic feed depending on format.
At first I was thinking it would be the "Hit Nation" feed (for CHR) or "Soft Rock" feed (or similar for AC),
but usually there's too many recurrents and "classic hits (so to speak)" on the overnight streams in both formats to be either of the more current-based national feeds/streams.
They're likely custom streams that run in the overnights and don't seem to be available via the iHeart streaming platform.
The Classic Rock station streams usually use the Doc Reno voice-tracked national feed that also appears on-air in that time period - otherwise I've occasionally heard the more generic "Classic Rock" feed.