Additionally, loneliness is associated with poorer sleep quality [48,49]; increases in depression and negative affect [5,24,50,51]; increased activation of the hypothalamus-pituitary-adrenal (HPA) axis, which plays a role in the body’s response to stress (particularly through increased release of glucocorticoids including cortisol [11,52,53]); and increased circulating levels of pro-inflammatory cytokines and inflammatory compounds (e.g., interleukin-6 (IL-6), C-reactive protein, and fibrinogen [11,54–58]). This evidence concerns the gene IL6 and major depressive disorder.