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A relationship exists between mood and cardiac control systems. This relationship has been established through correlations between medical pathology, such as Coronary Heart Disease (CHD), and psychopathological changes in mood, such as Major Depressive Disorder (MDD). Euthymic mood and normal cardiac regulation, as well as the diseases MDD and CHD, are linked by numerous interrelated physiological pathways. These pathways form part of a physiological regulatory network in the body called the psycho-immune-neuroendocrine (PINE) network. PINE network homeostasis can be disrupted by stress, resulting in pathological changes, which give rise to MDD, CHD, or both diseases. Heart Rate Variability (HRV) is a physiological measure of cardiac function which reflects autonomic cardiac control, an important component of the PINE network. This body of work set out to test three hypotheses, the first being that psychometric measures of mood and HRV measures are related across a broad range of mood, from non-pathological to pathological depressed mood. The second hypothesis is that psychometric measures of mood and HRV measures remain related over time, even when an individual’s mood changes across time. The third hypothesis is that in people with CHD, HRV measures would still be related to mood, but HRV may be globally reduced compared to people without CHD. A longitudinal cohort study, the Heart and Mind Study, recruited 88 participants and followed them up over six months.