Although orexin (hypocretin), melanin-concentrating hormone, adenosine, acetylcholine, dopamine, norepinephrine, histamine, gamma-aminobutyric acid (GABA), and glutamate are known to be involved in sleep-wake regulation [6], there is no clear evidence for a primary involvement of any of these systems in the pathophysiology of IH [7, 8]. Here, HCRT is linked to isolated hemihyperplasia.