Bipolar disorder is marked by fluctuating mood states over months to years. Which biological mechanism can give rise to these long fluctuations is not well understood. In a recent study, researchers combined longitudinal hair cortisol, individual hair cortisol measurements over a year, and mood measurements with mathematical modeling to provide a potential link between cortisol and mood timescales in bipolar disorder. Cortisol is a steroid hormone and when you are stressed, cortisol is released into your bloodstream.
Using 12 cm hair samples, representing a year of growth, researchers detected slow months-scale fluctuations of cortisol in participants with and without bipolar disorder. The amplitude of these fluctuations was 4-fold higher in bipolar than in controls. In addition, cortisol in the hair segment closest to the scalp positively correlated with recent mood scores—relating to the recent two months.
The researchers suggest a new mechanism in which daily stressors are perceived more strongly in bipolar disorder patients. These stressors excite slow timescales in the physiological system that control cortisol and stress, the hypothalamic–pituitary–adrenal (HPA) axis. The HPA axis is the system of organs in our bodies that control cortisol and generates elevated months-scale cortisol fluctuations, triggering mood episodes induced by prolonged high levels of cortisol.