Hypothalamic-pituitary-adrenal axis

HPA stands for hypothalamus, pituitary, and adrenal gland. These three organs pass signals along in sequence.

1. When you experience stress, the hypothalamus releases corticotropin-releasing hormone (CRH). 2. In response to this signal, the pituitary gland secretes adrenocorticotropic hormone (ACTH) into the blood. 3. When ACTH reaches the adrenal glands above the kidneys, they produce cortisol.

Cortisol raises blood glucose to provide energy, regulates inflammatory responses, and helps maintain blood pressure. The body needs energy and alertness to handle a crisis, and cortisol supplies the fuel, so to speak.

This hormonal pathway is not the only system involved in stress. Your heart races and your palms sweat during tense moments because the sympathetic nervous system responds first. The sympathetic nervous system alerts the body within seconds, while the HPA axis follows more slowly over several minutes to tens of minutes, sustaining the response. The two systems work together to complete the body's response.

When enough cortisol builds up in the blood, it sends the hypothalamus and pituitary gland a signal saying, "You can stop making it now." This is called negative feedback. Thanks to this process, cortisol levels gradually return to their original level after a stressful situation passes.

Activation of the HPA axis is fundamentally an adaptive response. It naturally switches on and off in brief, intermittent bursts. Allostatic load is a concept used to describe what happens when stress continues for a long time without an opportunity to recover. It refers to the wear and tear on the body caused by everyday minor events keeping the body's responses elevated for extended periods. Together with allostasis, it provides a framework for understanding how stress affects the body. Some research also suggests that childhood stress can have lasting effects on how a person responds to stress in adulthood.

• When you wake up in the morning. Cortisol is high in the morning and decreases as nighttime approaches. In particular, cortisol rises sharply for around 30 minutes after waking. This is called the cortisol awakening response. It helps the body wake up in the morning.
• Before an exam or presentation. You may feel rushed and queasy because this system is preparing your body.
• During an ongoing difficult period. Your sleep may become lighter, and your body may feel as though it is constantly tense. You can read more in Chronic stress and muscle tension.
• When you are short on sleep. Insufficient sleep may make your body more sensitive the next day. See also Sleep deprivation and pain sensitivity.

You cannot directly control the HPA axis, but lifestyle habits that give this system time to recover can help.

• Try to go to bed and get up at consistent times. Cortisol's daily rhythm is linked to the sleep–wake rhythm. Practicing sleep hygiene can help stabilize this rhythm.
• Try getting sunlight in the morning. Exposure to bright light during the day makes it easier to fall asleep at night.
• Keep moving regularly. Light exercise, such as walking, helps ease tension.
• Try calming your body with slow breathing. Slow, long exhalations activate the parasympathetic nervous system, easing tension.
• Build breaks into your daily routine. Your body needs time to relax to return to its original level.

Cortisol levels vary depending on the timing of blood or saliva collection, sleep, illness, and medications. For the cortisol awakening response immediately after waking in the morning, even a small deviation in collection time can change the result. International expert consensus guidelines therefore recommend following collection times precisely and recording conditions such as sleep and medication use. For this reason, it is best for test results to be interpreted in the context of the overall situation by a healthcare professional who knows when the sample was collected and the purpose of the test.