I have only grabbed some meaningful sleep on 5 flights.

LHR-JNB (x2)
JNB-LHR (x1)
JFK-LHR (x1)
ORD-FRA(x1)

On the first 4 they were overnight flights, late departure, so my body was naturally tired. Sat in a variety of cabins, from Y, to Y+ to J.

On the ORD-FRA we had a long ground hold for deicing and the booze was flowing freely in J. Coupled with a couple drinks at the AC and drink service during dinner I was fairly looped and passed out before desert. (I am not a heavy drinker, so 4 screwdrivers, 3 glasses of wine and 4 mimosas in a 4 hour period hit me hard) Oddly I had a great sleep and arrived in FRA feeling very good.