Not a good sleeper on planes. Temperature - as in too hot, and noise seem to be the biggest problems (and not talking about random noise like galley, screaming kids or the toilet, just the background engine noise, even with earplugs).
Actively avoid overnight flights if I can. Although for late night departures can usually get a few hours sleep from about 3 hrs into the flight (which is no good for short redeyes like PER-MEL). So very dependent on time of departure. If I'm on a 12 hr flight departing at 11am, I won't really get any sleep, as it is entirely during "normal" awake time, even if landing time is 6am in the destination.