Circadian and ultradian rhythms: Clinical implications

Abstract The hypothalamic–pituitary–adrenal axis is an extremely dynamic system with a combination of both circadian and ultradian oscillations. This state of ‘continuous dynamic equilibration’ provides a platform that is able to anticipate events, is sensitive in its response to stressors, remains robust during perturbations of both the internal and external environments and shows plasticity to adapt to a changed environment. In this review, we describe these oscillations of glucocorticoid (GC) hormones and why they are so important for GC‐dependent gene activation in the brain and liver, and their consequent effects on the regulation of synaptic and memory function as well as appetite control and metabolic regulation. Abnormalities of mood, appetite and metabolic regulation are well‐known consequences of GC therapy, and we suggest that the pattern of GC treatment and hormone replacement should be a much higher priority for endocrinologists and the pharmaceutical industry. One of the major impediments to our research on the importance of these cortisol rhythms in our patients has been our inability to measure repeated levels of hormones across the day in patients in their home or work surroundings. We describe how new wearable methodologies now allow the measurement of 24‐h cortisol profiles – including during sleep – and will enable us to define physiological normality and allow us both to develop better diagnostic tests and inform, at an individual patient level, how to improve replacement therapy.