Summary:
New research from the University of Bristol challenges the widely accepted belief that waking triggers the release of cortisol, the body’s primary stress hormone. The study found that cortisol levels increase during the hours before waking as part of the natural circadian rhythm, rather than as a response to waking itself. These findings, published in Proceedings of the Royal Society B, call for a reassessment of cortisol awakening response studies and their implications for conditions like sleep disorders, depression, and chronic fatigue syndrome.
Key Takeaways:
- Cortisol Increases Before, Not After, Waking: The study revealed that cortisol levels rise in the hours leading up to waking as part of the body’s circadian rhythm, rather than being triggered by waking itself.
- Challenges to Previous Research: Traditional studies of the cortisol awakening response relied on post-waking measurements, potentially misinterpreting the natural rise in cortisol as a response to waking.
- Implications for Health Research: These findings suggest that future studies on conditions like sleep disorders, depression, and chronic fatigue syndrome should focus on the interplay between cortisol dynamics and circadian rhythms rather than solely on waking.
Waking up does not activate an increase in the release of the stress hormone cortisol. Cortisol does, however, increase in the hours prior to awakening as part of the body’s preparation for the next day, new research led by the University of Bristol has found.
The study is published in the Proceedings of the Royal Society B.
For many years it has been generally accepted that waking up results in a stimulus to release 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.
Limitations of Traditional Cortisol Studies
A major limitation of studies using CAR is that protocols typically only assess samples obtained after waking up and not in the period prior to this since measurements are taken in saliva. Consequently, the studies are not able to prove a change in the rate of cortisol secretion over the awakening period.
To resolve the critical question of whether the rate of cortisol secretion actually increases after wakening, the Bristol research team used an automated sampling system to measure tissue cortisol levels both before and after wakening in 201 healthy male and female participants aged between 18 to 68 years old.
The researchers found awakening did not result in an increase in cortisol release, with no evidence for a change in the rate of cortisol increase in the hour after waking when compared with the hour prior to waking. This suggests that any change in cortisol levels immediately after waking are much more likely to be the tail end of the daily rhythm of cortisol—which starts increasing in the early hours of the morning and reaches a peak shortly after habitual wake time.
Implications for Understanding Circadian Rhythms
Importantly, the study also observed substantial interindividual variability in absolute concentration and rate of change and differences in dynamics that may be attributed to length and timing of sleep. Based on these findings, the researchers suggest caution is needed when interpreting cortisol measurements solely obtained in the hour after waking.
The findings demonstrate that the major cause of any changes in cortisol around the time of awakening are predominantly related to the endogenous circadian rhythm of cortisol. Furthermore, the results also suggest that if cortisol has any relationship to awakening, it is with factors that contribute to the initiation of awakening rather than being a response to it.
Future Directions for Cortisol Research
Circadian rhythms, the natural 24-hour cycles of physiological and behavioral patterns, are extremely important adaptations to living on our planet with its daily light/dark and temperature oscillations, and disturbances of these rhythms contribute to many psychological, metabolic, cardiovascular, and immunological health conditions. Understanding the role of cortisol rhythms in many of these conditions will be very important for researchers understanding of these disorders and their potential treatment.
“Our study opens up a whole new framework for understanding the relationship of overnight increases in cortisol with sleep and how this may be disrupted in sleep disorders, depression, and many other conditions,” Stafford Lightman, PhD, professor of medicine a Bristol Medical School: Translational Health Sciences and one of the lead authors of the study, in a release.
The research team suggests that future studies on mechanisms of arousal from sleep both overnight and during the morning should carefully consider dynamic changes in the activity of the hypothalamic pituitary axis—the system in the body that regulates the stress response and the release of cortisol—in addition to sleep and behavior.
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