The Cortisol Awakening Response: A Biological Morning Surge
In 1997, researcher Jens Pruessner and his colleagues made a fascinating discovery while analyzing saliva samples. They noticed a remarkable hormonal pattern unrelated to whether a person was an early riser or a night owl. Within the first thirty to forty-five minutes after waking, cortisol levels rise sharply — by an average of 38 to 75 percent — in nearly everyone studied. This surge, now known as the cortisol awakening response (CAR), occurs relative to the exact moment a person wakes, not according to the clock. Whether you rise at 5 a.m. or 11 a.m., the surge happens on schedule.
This distinction is more intriguing than it might first appear. Much of the conversation around mornings, including my own recent exploration of the genetics behind being a lark or an owl, centers on timing — when you wake and whether that timing can be shifted. The CAR tells a different story. It focuses on what your body does in the first 45 minutes after waking, regardless of clock time. What follows is a summary of current research rather than medical advice; anyone experiencing chronic stress or exhaustion should consult a healthcare professional.
What Happens in the First 45 Minutes After Waking?
Cortisol, often called the “stress hormone,” follows a daily rhythm. It rises gradually during the latter stages of sleep and falls throughout the day, reaching its lowest levels near bedtime. The CAR is a distinct, sharper spike superimposed on this rhythm, peaking roughly 30 minutes after awakening and then tapering off. This phenomenon has been observed across all ages, from infants to the elderly, and has generated hundreds of scientific studies annually at its peak.
While the exact function of the CAR remains under investigation, the leading theory — known as the preparation hypothesis — suggests it primes the brain and body for the day’s upcoming demands. Rather than reacting to past events, the CAR acts as a proactive biological readiness signal, equipping you to face whatever challenges the day holds.
It’s Not Just About Light
A common assumption is that morning light triggers the CAR. However, research challenges this view. Studies have shown that waking earlier correlates with a larger CAR — the opposite of what you’d expect if light intensity were the main driver. Notably, researchers Kudielka and Kirschbaum, renowned for their extensive work on CAR, concluded that the response is not simply a reaction to light exposure.
Genetics also contribute to the CAR independently of chronotype (whether you’re naturally a morning or evening person). Twin studies estimate the heritability of the CAR’s magnitude at around 40%, indicating that biological factors influence both when you prefer to wake and how vigorously your body responds in the first moments after waking.
Why the CAR Might Influence Cognitive Performance Later in the Day
One of the most compelling areas of CAR research involves its impact on brain function hours after waking. In one study, participants with a stronger morning CAR exhibited a distinctive brain activity pattern during challenging working memory tasks in the afternoon. Specifically, they showed reduced activation in the hippocampus and prefrontal cortex but stronger connectivity between these regions, which corresponded with better task performance.
A pharmacological follow-up study confirmed a causal relationship by manipulating cortisol levels experimentally, rather than merely observing natural variations. If the preparation hypothesis holds true, this suggests that the CAR during the first 45 minutes after waking plays a meaningful role in how the brain manages cognitive load later — independent of external factors like caffeine or breakfast routines.
The Double-Edged Sword of Cortisol
It’s important to avoid oversimplifying the CAR as a universally “good” surge. Elevated CAR levels have been linked to job stress and general life stress, while a reduced or blunted CAR is associated with fatigue, exhaustion, and burnout. This means that the same hormone surge that prepares the brain might also reflect how much strain a person is already under. In other words, the CAR acts as a sensitive marker that fluctuates with life circumstances, rather than a dial with a single optimal setting.
If feelings of persistent stress or exhaustion resonate, seeking help from a healthcare provider is the best course of action, rather than relying on generalized research findings.
What Can We Take Away?
While the CAR doesn’t offer a new morning ritual or a magic bullet for productivity, it enriches our understanding of mornings beyond simple wake-up times or routines. There is a complex hormonal process quietly unfolding during everyone’s first 45 minutes awake — whether you’re a 5 a.m. riser or an 11 a.m. sleeper. Scientists are still unraveling its purpose, but acknowledging this biological nuance encourages a more compassionate and less judgmental view of morning productivity and personal rhythms.
For those curious to dive deeper into the science behind this phenomenon, further reading is available Here.
