BDNF and insomnia: The present study, therefore, aimed to explore the relationship between insomnia and a diverse selection of immunologic (e.g., tumour necrosis factor-α receptor 2 and calprotectin), neurotrophic (e.g., brain-derived neurotrophic factor and epidermal growth factor), neuroendocrine (e.g., cortisol and aldosterone), neuropeptide (i.e., substance P), and metabolic (e.g., leptin and acetyl-L-carnitine) biomarkers in a clinical sample of individuals with MDD.