I guess, to be clear, we are talking about US domestic (mainland) or sub-8-hour TATL (e.g., JFK-LHR).

I used to love redeye, but no longer. Even on long routes like LAX-BOS, it's still not enough time to sleep and wake up groggy. Lie-flats help (thanks, B6), but still only get less than 5 hour of sleep and not primed for work the same day.