Cortisol research challenges previous assumptions

Waking up does not trigger an increase in the release of the stress hormone cortisol. However, cortisol increases in the hours before waking as part of the body’s preparation for the next day, according to new research led by the University of Bristol. The study was published on January 15 in the Proceedings of the Royal Society B.

For years it has been widely accepted that waking up results in a stimulus to release the hormone cortisol – a phenomenon called the ‘cortisol awakening response’ (CAR). This response has been used to investigate many clinical conditions, including PTSD, depression, obesity and chronic fatigue syndrome (ME/CFS).

A major limitation of studies with CAR is that protocols typically only assess samples obtained after awakening, and not in the period before, as measurements are performed in saliva. Consequently, the studies are unable to prove a change in the rate of cortisol secretion during the awakening period.

To resolve the crucial question of whether the rate of cortisol secretion actually increases after waking, the Bristol research team used an automated sampling system to measure tissue cortisol levels both before and after waking in 201 healthy male and female participants between 18 and 68 years old. .

The researchers found that waking did not result in an increase in cortisol release, with no evidence of a change in the rate of cortisol increase in the hour after waking compared to the hour before waking. This suggests that any change in cortisol levels immediately after waking is much more likely to be the tail end of the diurnal rhythm of cortisol, which begins to increase in the early hours of the morning and peaks shortly after the usual waking time.

Importantly, the study also observed substantial inter-individual variability in absolute concentration and rate of change, and differences in dynamics attributable to the length and timing of sleep. Based on these findings, the researchers suggest that caution should be used when interpreting cortisol measurements obtained only in the hour after waking.

The findings show that the main cause of any changes in cortisol around the time of waking is mainly related to the endogenous circadian rhythm of cortisol. Furthermore, the results also suggest that if cortisol has any relationship to awakening, it is to do with factors that contribute to the ‘initiation’ of awakening, rather than being a ‘response’ to it.

Circadian rhythms, the natural 24-hour cycles of physiological and behavioral patterns, are critically important adaptations to life on our planet with its daily light: darkness and temperature fluctuations, and disruptions to these rhythms contribute to many psychological, metabolic, cardiovascular and immunological health problems . conditions.

Understanding the role of cortisol rhythms in many of these conditions will be of great importance to researchers’ understanding of these conditions and their potential treatment.

Stafford Lightman, Professor of Medicine at Bristol Medical School: Translational Health Sciences (THS), and one of the lead authors of the study, said: “Our study opens up a whole new framework for understanding the relationship between nighttime increases in cortisol and sleep. . , and how this can be disrupted in sleep disorders, depression and many other conditions.”

Dr. Thomas Upton, Clinical Research Fellow at Bristol Medical School (THS) and co-lead author, explained: “By measuring both before and after waking, this study provides much-needed and crucial insight into the dynamics of cortisol in relation to sleep and endogenous rhythms, an important message to me is that great caution should be exercised when attempting to interpret post-awakening cortisol levels when information about the pre-awakening state is unknown.”

Marcus Munafò, Professor of Biological Psychology and Associate Pro Vice-Chancellor – Research Culture at the University of Bristol, added: ‘This work not only provides important insights into the biology of our sleep-wake cycles, but also illustrates how findings received The wisdom within the research community may be wrong.

“Ensuring that our work is robust and reproducible – including rigorous testing of previous findings – is a central part of the research culture we seek to promote at the University of Bristol.”

The research team suggests that future studies examining mechanisms of arousal from sleep, both nighttime and morning, should carefully consider dynamic changes in the activity of the hypothalamic-pituitary axis (the system in the body that controls the stress response and the regulates cortisol release). in addition to sleep and behavior.