Chances are the "mellow" that you are feeling from this is all in your head. Most poppies do not have opium, and chances are the poppy seeds you pick up at the grocery store are NOT opium poppies. It is illegal(in the US, don't know about elsewhere) to grow Papaver somniferum (opium poppies) but other poppies are perfectly legal. This is because other species of poppy don't contain opium.
http://www.erowid.org/plants/poppy/ for more info about opium poppies