Yes it seems bizarre but that is because with lower doses GHB affects other receptors: GHB-receptors that are stimulating. In higher doses this shifts to GABA action, it might be that this shift is so concentration-critical that it is the reason for the relatively small 'therapeutic index'.
When you have taken a high dose and recover from it, the levels of GHB in your body eventually drop until they are like a low dose again, which is why you get a stimulant effect. Also, since GABA action seizes there might be mild acute withdrawal / paradoxical effects translated through monoamine or glutamate action.
So for narcolepsy the doses would be lower than for sleep induction. I would venture a guess that a good dose for narcolepsy would be half of a recreational dose that approaches a KO point. So that would be what? 1 gram? I am used to calculating with saturated solutions though... 4 ml is a whole dose so 2 ml half.
But I would be wary about fucking around with GHB without supervision, especially when talking about chronic medication. If you do such a thing I predict that the way to do it is never increase or decrease your doses, and expect discontinuation withdrawal, which for a 'half a dose' habit should be quite transient and manageable.
Again though: consult a physician.
How is this related to psychedelics though?? No idea though if you rather want this in OD/BDD, I think Other Drugs covers pharmaceuticals. Which GHB is, in this particular form and application of xyrem.
http://en.wikipedia.org/wiki/Narcolepsy#Treatment
I would also keep modafinil in mind. I have that stuff myself and really appreciate that it promotes alertness without having the load or charge of feeling as if really on a drug. It is very clean and to the point. For AD(H)D it might still be controversial, but with something like narcolepsy if it were me I'd rather see if modafinil catches on than the ritalin/amphetamine kind of stuff. The actual stimulants as opposed to mere wakefulness promotors.