This article gives a great summary of recent studies into the vagus nerve and its role in health as the “rest and digest” side of the autonomic nervous system.
Clinical and experimental studies indicate that stress and depression are associated with the up-regulation of the immune system, including increased production of pro-inflammatory cytokines…When administered to patients or laboratory animals, cytokines have been found to induce typical symptoms of depression. Therefore some cases of low mood, low energy, and lack of motivation may be due to elevated levels of cytokine proteins.