It wouldn't be giving shamans sleep. That would not be fair in the least. It would give shamans an ability to chase, not that they can't already. It was just a random idea and it makes more sense than insomnia, because the way it is right now, it doesn't really make a big impact unless you get away. I am also unsure of this other spell you were talking of that is nearly identical...if you speak of insomnia, it is the complete opposite, if you talk of sermon, it is for clerics, and if you talk of something else, I'm out of the loop.
I also do not like the idea of it doing anything OTHER than forcing the character to go to sleep and waking up as soon as they type "st". I think this is a fun idea because it can totally screw someone's running commands up and allows the shaman to catch up to someone faster than them. What I think it SHOULD do is cause the person to be hit for the other things first, and then fall asleep. So it would go poison, plague, that other spell, and then make them fall asleep, and able to wake up right away.
I do not think clerics deserve this spell, they are fine the way they are, aside from the ray vs path that everyone debates. I do not mandate that if this spell goes in, it goes in on shamans. There are other classes this could go on. It might be neat on invokers, psionicists, liches, anything. You know? It could even be given to a melee class in the form of a melee attack to the head. It would last for a few hours and if you are trying to run, oops, you get whoosy and fall unconscious for a split second.