By that logic, I've flown a ton of redeye ATL-MIA (11pm to about 12:30 the next morning).

Probably bringing too much logic to this... yet... to me, a redeye is a flight that departs toward the end of one daily flight-cycle, and lands after the beginning of another daily flight cycle. At least under NW, daily flight cycles appear to have ended/started around 2-3am CST, and the nightly processing (including FF account updates) would happen around this time. IIRC, there were some LAX/LAS red-eyes that would depart as late as 1:30am CST, and other midwest/east-coast flights that would land as early as 3:30am CST. Flights that crossed that time-window I'd consider redeyes.

On the other hand, if you don't know the technical answer form the airline's ops department, I'd consider it like art... you'll know it when you see it. To me, it's a flight that's last-out on the west-side, and first-in on the east-side.