Biomarkers of Mental Illness—What Can We Learn from Circadian Heart Rate?

Background: Much research in psychiatry has been a search for diagnostic biomarkers of mental illness but practically useful markers have remained elusive. The problem may be unrealistic expectations and the aim in this paper is to show that the relationship between circadian heart rate and psychiatric status can contribute to useful understanding in this regard. Aim: To discuss the biomarker implications of changes in circadian heart rate (CHR) in psychiatric disorders. Methods: Comparisons of CHR were made between and within individuals receiving treatment for different psychiatric disorders diagnosed according to criteria defined in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5). Results: Broadly different DSM-5 disorders are associated with distinctly different changes in CHR. Some disorders are more consistently associated with distinctive changes but CHR does not predict symptoms or specific clinical diagnoses reliably. Changes in CHR, particularly during sleep, are state-dependent. Clinical improvement is associated with normalisation of CHR. Conclusion: Changes in CHR are a part of the physiological changes in mental illness. Distinctly different changes in CHR suggest distinctly different physiological changes that may constitute diagnostic discrimination at a physiological level. An analysis of CHR can add objective adjunct information to clinical assessment and the evaluation of treatment but does not predict symptoms or clinical diagnoses reliably. Much the same is likely to apply to all candidate biomarkers of mental illness.