This is such a silly question.... Jupplypuff's song is called a lullaby. Lullabys are melodys that are used to put people to sleep, works especially well on children. Ever see a parent sing their child a song before they go to sleep? They are supposed to calm and sooth your body and mind, which eventually puts you to sleep because of the comfortable feeling you get listening to them... Of course, not just lullubys do this, other calming songs or songs that you like can put you to sleep, as long as it makes you comfortable. Jigglypuff's song puts people to sleep just like that, though I'm sure there is a magic to it as well thats only found in the relm of pokemon.
The reason why it doesn't affect you is because... well... Pokemon is an anime, and just like any anime, it tends to over exagerate many many things. Real people can't survive a hyper beam or a thunder bolt like that. Just like an entire city can't be put to sleep from one little song in reality.
Thats my reasoning anyway... feel free to think of a different one. ^_^