Narcolepsy is a sleep disorder caused by impaired hypocretin (orexin) neurotransmission. Growth hormone (GH) secretion may be altered in narcolepsy for various reasons. Slow wave sleep episodes, which are closely associated with GH secretory events, are more randomly dispersed over 24 hours in narcoleptics. Furthermore, hypocretins may inhibit pituitary GH release. We assessed the function of the somatotropic axis in narcolepsy, by deconvolving 24- hour (10 minute sampling interval) plasma GH concentration profiles in 7 hypocretindeficient narcoleptic patients and in 7 healthy controls matched for age, sex and body weight. Both basal and pulsatile GH secretion rate and secretagogue-induced GH release were similar in patients and controls. However, narcoleptics secreted approximately 50% of their total production during daytime, whereas controls secreted only 25% during the day. Also, the GH output pattern of narcoleptics was significantly less regular. We propose that hypocretin deficiency disrupts the circadian distribution of hypothalamic GHRH release in narcoleptic patients to simultaneously cause daytime GH release and promote their propensity to fall asleep during the day.