The stress-related hormone cortisol is of central importance as a biomarker of mental health and has significant downstream effects on physical function. Most cells and organs in the body possess receptors for glucocorticoid hormones, including cortisol. Glucocorticoids are known to cross the blood-brain barrier, and impact behavior through limbic influence. Glucocorticoids are also key regulators of immunity and the inflammatory response, and cortisol levels influence energy metabolism, appetite, and visceral adiposity.
There can be enormous variability in diurnal cortisol rhythms, inter-individually as well as intra-individually. Daily cortisol levels are usually highest around waking time, fall quickly over the next few hours, and generally reach a minimum around bedtime. Earlier research into cortisol rhythms focused on basal levels, whereas more recent work demonstrates that circadian rhythmicity may better reflect the long-term effects of cortisol regulation on health and immune function. Three main components of the diurnal cortisol rhythm are daily average cortisol, the magnitude of the cortisol awakening response, and the diurnal cortisol slope (DCS).
Diurnal cortisol slopes depict changes in cortisol level from morning to evening, and flatter DCSs are theorized to bridge chronic psychosocial stress with poorer mental and physical function. For this meta-analysis, researchers studied associations between DCS and twelve categories of health outcomes: anxiety, depression, disorders of internalization of feelings, disorders of behavioral externalization of feelings (such as anger and aggression), fatigue, obesity, cardiovascular illness, cancer, other physical and mental health outcomes, and mortality.