Chronic widespread pain (CWP) is one of the most difficult conditions to treat due to its unclear aetiology and multidimensional sensory-discriminative, motivational-affective, and cognitive-behavioural nature. However, current findings within the fields of autoimmune dysregulation, parasympathetic dysregulation, and neurocognitive-affective syndromes suggests three domains in which chronic pain may be successfully treated. This study will be the first of its kind to use a randomised, controlled, two-stage experimental design to investigate and compare the efficacy of non-invasive transcutaneous vagal nerve stimulation (tVNS) and diaphragmatic breathing (DB) for the treatment of CWP. We hypothesise that both tVNS and DB will significantly increase ECG-measured high-frequency heart rate variability (HRV) and therefore increase pain inhibition and vagal parasympathetic tone while decreasing pro-inflammatory sympathetic overactivity (as indicated by hair cortisol levels and pro-inflammatory markers) in participants with CWP. Patients randomised to the tVNS treatment will undergo a 60-minute low- frequency stimulation once a day for 6 weeks while patients randomised to the DB program will practice specific deep breathing exercises for a duration of 60 minutes per day for 6 weeks.