I use a brand I purchased at a pharmacists in the USA called "Unisom" - its a little blue pill that contains, I seem to recall, the ingredient in anti-histamines that makes you drowsy - it certainly knocks me out for a good 5 hours (I really struggle to sleep on planes so 5 hours is amazing for me)......albeit I feel drowsy /dopey for a good 4 hours when I wake up.....

Might be good for those looking for a longer sleep......though not if you have n important meeting on arrival!