Stress hormones rise during sleep to prepare your body for the day

For years, it was widely believed that waking up triggered an immediate release of cortisol, the stress hormone.

This response, known as the cortisol awakening response (CAR), has been used in clinical studies. However, new research from the University of Bristol reveals that this common assumption might be incorrect.

The study suggests that cortisol levels actually increase in the hours before waking up, not immediately upon waking. The research challenges previous conclusions that linked cortisol release directly to the awakening process.

Cortisol awakening response

Previous studies have relied on the concept of CAR to evaluate how the body responds to stress in conditions like PTSD, depression, and obesity. The CAR assumes that cortisol, a stress hormone, spikes right after waking as a natural response to the stress of starting the day.

However, these earlier studies focused only on measuring cortisol levels in saliva after the person had already woken up, missing an important part of the process – the time before waking. This left an incomplete picture of how cortisol levels change during the sleep-to-wake transition.

To address this gap, researchers at the University of Bristol designed a study to measure cortisol levels both before and after waking. Instead of relying on post-wake saliva samples, they used an automated sampling system to track cortisol in 201 participants aged 18 to 68.

This allowed them to analyze the full pattern of cortisol secretion, including the changes that occur before waking up.

Cortisol does not spike after waking

The results were surprising. The study found no evidence that cortisol secretion increases immediately after waking. In fact, there was no significant difference in the rate of cortisol increase during the hour after waking compared to the hour before waking.

“We found that any changes in cortisol around the time of awakening are more likely to be the tail end of the daily rhythm,” said Stafford Lightman, professor of medicine at Bristol Medical School and one of the study’s lead authors.

This rhythm begins in the early hours of the morning and peaks shortly after habitual wake time.

Role of circadian rhythms

The study also uncovered substantial variability in cortisol concentration and its rate of change among individuals.

Factors such as sleep length and timing seemed to influence these differences. The researchers cautioned that interpreting cortisol values solely based on post-wake measurements could be misleading.

Dr. Thomas Upton, clinical research fellow and co-lead author, emphasized the importance of considering both pre- and post-wake cortisol levels in future studies.

“Much caution should be exercised if attempting to interpret post-wake cortisol values where information about the pre-waking state is not known,” he explained.

The findings also stress the importance of circadian rhythms, the body’s natural 24-hour cycles, in regulating cortisol. “Cortisol changes around waking are more about the body’s internal rhythms than a response to awakening,” said Lightman.

Implications for health conditions

The research offers valuable insights into the role of cortisol in sleep disorders, depression, and other conditions.

“Our study opens up a whole new framework for understanding the relationship of overnight increases in cortisol with sleep,” Lightman said.

Marcus Munafò, professor of biological osychology at the University of Bristol, noted that this study challenges “received wisdom” in research and stresses the importance of rigorously testing previous findings.

Professor Munafò also highlighted the significance of the study in understanding the biology of sleep-wake cycles.

Future research on stress hormones

The researchers suggest that future studies on sleep and arousal should examine the dynamic changes in the hypothalamic-pituitary axis, which regulates cortisol release.

Understanding how sleep affects cortisol secretion could lead to better insights into various health conditions and improve treatment strategies.

The study was funded by the EU Horizon 2020 grant ‘Ultradian’ and the Biotechnology and Biological Sciences Research Council (BBSRC).

Ultimately, the new findings encourage a reassessment of the cortisol awakening response and emphasize the complexity of cortisol regulation.

Researchers will need to consider both pre-wake and post-wake cortisol levels to gain a fuller understanding of how this hormone interacts with sleep, health, and behavior.

The study is published in the journal Proceedings of the Royal Society B Biological Sciences.

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