I use this one, when I REALLY have to wake up, QUICK! I pull my tits out of my top (or just remove my top altogether),and open it wide, position it over my nipple, and just let it SNAP! closed, right on my nipple! (Mind you, I found I have GOT to pull over before I do this! I tried ONCE, to do this while driving, and almost drove off the road!) I do one nipple, and start driving, and after mile or two, I pull over, and do the other nipple!
The pain/pleasure quotient on these is really great, because I get that great rush of endorphins from the pain, which keeps me awake, while the pleasure starts to really build up!