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Background: Markers of HPA axis function are increasingly reported as disrupted in schizophrenia (SZ) and bipolar disorder (BD). These markers include aberrant diurnal cortisol rhythm, and/or cortisol responses to stress or pharmacological manipulation in SZ and BD, while associations between cortisol dysfunction and illness characteristics remain unclear.

Methods: In this study, we examined cortisol levels of SZ and BD participants, relative to healthy controls (HC), using spline embedded linear mixed models, to determine group differences (a) at waking,(b) 45 minutes after waking (with the difference between a and b representing the Cortisol Awakening Response; CAR),(c) during the post-CAR decline, and (d) in reaction to a stressor (MRI scan).

Results: In SZ, the cortisol response to the MRI stressor showed a significant absence of the expected increase in cortisol response to stress that was seen in both the …