Another +1 for the day flight.

When coming back from the US, I find JFK-LHR to be of such a length that, regardless of cabin, you only get enough sleep to realize how much more sleep you still need. You still have to stay awake for take-off & landing, etc.

If one HAS to do the overnight ex-JFK, and sleep is a priority, I'd plan on dinner either in NYC or at the lounge, I'd skip dinner & breakfast on the plane, and grab something in the arrivals lounge at T-5.