I have to be tired; really tired...to the point my body passes out. There are so many things that make sleeping on a flight next to impossible for me:
1) I'm a side sleeper - couch seating isn't comfortable.
2) If in FC; all the racket from dining utensils.
3) Tried noise-cancelling headphones...again, side sleeper; headphones aren't comfortable.
4) Short - need to have my legs elevated. Yes, I have placed bag under feet.

I'm one of those unfortunate souls who can't sleep on planes. Boo Hoo.