In the MDD group, patients with higher SI had higher hypocretin-1 levels than those with lower SI [62.09 (38.23–80.12) pg/ml vs. 56.63 (35.79–77.37) pg/ml, P = 0.032, see Figure 1B], but this difference was not found in the BD group (P = 0.926). Here, HCRT is linked to major depressive disorder.