It's not just you. I get into that seat and bang, sleepy times. I've slept through take off in the past. Yet to figure out how I slept through the sound of two CFM56s roaring away when I can't sleep at home without complete silence.
I think it's a combination of relaxing as I'm on my way and the early starts that flying seems to demand. Up at 4am for a 9:30 flight... 2 hour drives are fun at that time.