Cleavage product (AA 70 – 97) of the 161 aminio acid propeptide orexin coded by the HCRT gene. It binds to the orexin receptor type 2 with a high affinity and so has a role in the regulation of sleep. Some mammals such as cattle get by with only effectively orexin-A. Absent or very low levels are found in the cerebrospinal fluid in most patients with narcolepsy. Orexin-2 receptor antagonists initiate sleep and prolong sleep in the sleep-awake structures of the lateral hypothalamus.