Rethinking the cortisol awakening response: Is waking up that stressful?

For decades, scientists have believed that waking up triggers a surge in cortisol — a hormone that helps regulate stress, metabolism, and immune function. This phenomenon, known as the  “cortisol awakening response” (CAR) has been widely used in research on mental health conditions like PTSD, depression, and schizophrenia, as well as chronic illnesses such as obesity and chronic fatigue syndrome. However, a new study published in January 2025 challenges this long-held assumption. The rise in cortisol seen after waking may not be a response to waking at all — but rather part of the body’s natural rhythm.

The assumption that waking triggers cortisol release comes from research studies that never directly measured cortisol before waking. Most research on CAR relies on saliva samples collected only after waking, making it hard to tell whether cortisol levels increase because of waking or simply follow a daily pattern. To fill this gap, a team of researchers at the University of Bristol used an advanced ambulatory microdialysis technique to track cortisol levels both before and after waking in participants. 

Their results? Waking up itself doesn’t seem to cause a spike in cortisol. Instead, cortisol levels start rising before wake time and peak shortly after, following a pattern set by the body’s internal circadian rhythm. 

“Waking up itself doesn’t seem to cause a spike in cortisol. Instead, cortisol levels start rising before wake time and peak shortly after, following a pattern set by the body’s internal circadian rhythm.”

The circadian rhythm is an internal 24-hour cycle that regulates sleep, hormone release, and metabolism and is responsible for many physiological processes. This study suggests that the morning rise in cortisol is part of the body’s natural preparation for waking, not a response to it. 

Previous research supports this idea. Studies have found that cortisol increases only when waking aligns with the body’s natural cortisol peak. If someone wakes up at a time when cortisol is normally declining, like after an evening nap, there’s no post-wake cortisol spike. Similarly, people who wake up earlier in the morning tend to have higher cortisol levels simply because they’re waking during a phase of their circadian rhythm when cortisol is naturally rising.

An interesting finding in this study is the significant variability between individuals’ cortisol levels and secretion times. Some people experience a steady rise in cortisol after waking, while others peak before waking or show little change at all. Factors like sleep duration, wake timing, stress levels, genetics, and even light exposure can all influence cortisol patterns. This means that cortisol measurements taken after waking may not give a full or accurate picture of an individual’s stress response. It also explains why some studies have found wildly different CAR results, depending on when and how participants woke up. 

If cortisol levels follow an internal rhythm rather than responding to waking, it raises important questions for research on stress and mental health. Many studies have used post-wake cortisol levels as a measure of stress regulation, assuming that a blunted or exaggerated CAR reflects a person’s overall well-being. However, if waking isn’t the trigger, then those studies’ conclusions may need to be reconsidered. 

Instead of focusing on waking as a cortisol driver, future research should look at how sleep patterns, habitual wake times, and broader circadian influences shape cortisol fluctuations. This shift in perspective could lead to better measurements of stress and developing treatments that align with the body’s natural rhythms whether in mental health care, sleep medicine, or beyond.