Medical Definition of orexin
: either of two neuropeptides ( orexin A and orexin B ) that are produced in the hypothalamus and play a role in regulating wakefulness and food intake Scientists have known for almost a decade that narcolepsy is caused by a dearth of a brain communication chemical called orexin.— Laura Sanders, Science News, 24 Oct. 2009 The new sleep aids block the activity of brain peptides called orexins.— Christine Soares, Scientific American, July 2010
— called also hypocretin
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