The cortisol awakening response (CAR) is a natural increase in cortisol that occurs in response to waking in the morning. Normally cortisol is highest in the morning, shortly after waking, and gradually decrease throughout the day. The CAR is a sharp increase in cortisol that occurs within the first 30-45 minutes after waking, typically peaking around 30 minutes. This increase in cortisol levels is thought to be a natural response to the body's need to mobilise energy and prepare for the demands of the day.

The CAR has been studied as a marker of the body's response to stress and has been associated with various physical and mental health outcomes, such as depression, anxiety, and chronic fatigue syndrome. Researchers have also explored the relationship between the CAR and sleep quality, with some studies suggesting that disrupted sleep patterns may interfere with the CAR.