The following keywords were used: (Schizophrenia OR schizophrenic disorders OR dementia praecox OR bipolar disorders OR bipolar mood disorder OR depression OR depressive disorder) AND (orexin OR hypocretin OR orexin-A OR hypocretin-1 OR orexin-B OR hypocretin-2) AND (plasma OR serum) AND (measurement OR levels). Here, HCRT is linked to major depressive disorder.