HCRT and major depressive disorder: This systematic review included (a) adult patients diagnosed with schizophrenia, MDD, or bipolar disorder (DSM-IV/V or ICD-10); (b) studies with healthy control groups; (c) studies measuring plasma orexin-A levels; (d) randomized controlled trials or observational studies (cross-sectional, cohort, case-control); and (e) studies published in English.