Cortisol is a steroid hormone produced by your adrenal glands that regulates metabolism, immune function, blood pressure, and your body’s response to stress. It’s often called the “stress hormone,” but that label undersells it. Cortisol is active in nearly every organ system, and your body needs a steady, well-timed supply of it just to function normally.
How Your Body Makes Cortisol
Cortisol production starts in the brain, not the adrenal glands. When your brain detects a threat or stressor, a region called the hypothalamus releases a signaling hormone that travels to the pituitary gland at the base of the skull. The pituitary responds by releasing ACTH into your bloodstream, which travels to the adrenal glands sitting on top of your kidneys. ACTH tells the adrenal cortex to convert cholesterol into cortisol, which is then released into general circulation where it can reach receptors in virtually every organ system, including the brain itself.
This chain of communication, called the HPA axis, has a built-in off switch. Once cortisol levels rise high enough, the hormone signals back to the hypothalamus and pituitary to stop producing their respective triggers. This negative feedback loop keeps cortisol within a normal range under healthy conditions.
Cortisol’s Daily Rhythm
Cortisol doesn’t stay at a flat level throughout the day. It follows a pronounced daily cycle: levels are highest in the early morning and lowest around midnight. Within the first 30 to 45 minutes after you wake up, cortisol surges an additional 38% to 75% above your waking level. This burst, known as the cortisol awakening response, helps mobilize energy and alertness for the day ahead. From that morning peak, levels gradually decline through the afternoon and evening, reaching their lowest point during the first few hours of sleep.
Disruptions to this rhythm, whether from shift work, chronic stress, or sleep disorders, can have cascading effects on metabolism, mood, and cognitive function.
How Cortisol Controls Blood Sugar
One of cortisol’s most important jobs is keeping your blood sugar available when your body needs fuel. It does this through several coordinated mechanisms. In the liver, cortisol activates enzymes that produce new glucose from non-sugar sources like amino acids and glycerol, a process called gluconeogenesis. At the same time, it promotes the breakdown of protein in muscle tissue and fat in adipose tissue, freeing up those raw materials for the liver to convert.
Cortisol also works against insulin in muscle and fat cells. It reduces the number of glucose transporters that move to cell surfaces in response to insulin, meaning those tissues absorb less sugar from the blood. It further blocks glucose from being burned for energy inside cells by interfering with a key step in glucose oxidation. The net effect: more sugar stays in the bloodstream, available for the brain and other critical organs during a crisis. This system works well in short bursts. When cortisol stays elevated for weeks or months, the same mechanisms can lead to chronically high blood sugar and insulin resistance.
Effects on the Immune System
Cortisol is one of the most potent natural anti-inflammatory substances your body produces. Because it’s a small, fat-soluble molecule, it passes easily through cell membranes and binds to receptors inside immune cells. Once activated, these receptors suppress a master inflammatory switch called NF-kB, which normally drives the production of inflammatory signaling molecules in nearly all immune cell types. Cortisol also blocks other inflammatory pathways and suppresses the activity of T cells, which are central to adaptive immunity.
This is why synthetic versions of cortisol (corticosteroids like prednisone) are prescribed for conditions involving overactive immune responses, from asthma to autoimmune diseases. It’s also why people under chronic stress tend to get sick more easily. Persistently elevated cortisol can suppress immune surveillance enough to leave you vulnerable to infections, while paradoxically, the immune system can eventually become less responsive to cortisol’s calming signals, allowing runaway inflammation in some cases.
Blood Pressure and Cardiovascular Effects
Cortisol helps maintain blood pressure by making blood vessels more responsive to adrenaline and noradrenaline, the hormones that constrict blood vessels during a stress response. It does this from two angles. In the cells lining blood vessels, cortisol suppresses production of natural vasodilators like nitric oxide and prostacyclin, substances that normally relax vessel walls. In the smooth muscle cells of vessel walls, it enhances the mechanical coupling that causes constriction. Without adequate cortisol, blood vessels don’t respond properly to adrenaline, which is why people with severe cortisol deficiency can experience dangerously low blood pressure.
What Chronic High Cortisol Does to the Brain
Short-term cortisol spikes sharpen alertness and help form memories. Prolonged elevation does the opposite. The hippocampus, the brain region central to learning and memory, is densely packed with cortisol receptors, making it especially vulnerable to chronic exposure. Animal studies have shown that sustained high cortisol leads to hippocampal shrinkage and neuronal damage. In humans, research on patients with Cushing’s disease (a condition of pathological cortisol excess) confirms that prolonged exposure impairs memory, reduces neuroplasticity, and can cause measurable brain atrophy.
The prefrontal cortex, responsible for executive function, planning, and attention, is also affected. So is the amygdala, the brain’s emotional processing center, which can become overactive under chronic cortisol exposure. This combination helps explain why people under sustained stress often experience difficulty concentrating, poor memory, and heightened anxiety or emotional reactivity. Disrupted cortisol rhythms, particularly the loss of normal nighttime lows, may accelerate these changes by preventing the brain from undergoing its normal restorative processes during sleep.
When Cortisol Is Too High
The most common cause of pathologically high cortisol isn’t a disease. It’s the long-term use of corticosteroid medications prescribed for inflammation and autoimmune conditions. When the body itself overproduces cortisol, the condition is called Cushing’s syndrome. In 8 out of 10 non-medication cases, the cause is a noncancerous pituitary tumor that produces too much ACTH. Less commonly, tumors in the lungs, pancreas, thyroid, or adrenal glands themselves can drive excess production.
The signs of Cushing’s syndrome reflect what happens when cortisol’s normal functions go into overdrive. Weight gain concentrates in the face, upper back, and abdomen while arms and legs become thinner as muscle wastes away. Wide purple stretch marks appear on the torso. Skin bruises easily. Women may develop excess facial and body hair and irregular periods. Men may experience reduced fertility and sex drive. Children with the condition tend to grow more slowly than their peers while gaining weight.
When Cortisol Is Too Low
Addison’s disease, or primary adrenal insufficiency, occurs when the adrenal glands can no longer produce enough cortisol (and usually aldosterone, a related hormone that controls sodium and potassium balance). Without adequate cortisol, the body loses its ability to maintain blood sugar between meals, regulate blood pressure, mount an appropriate stress response, and control inflammation. Symptoms include profound fatigue, muscle weakness, weight loss, low blood pressure, and sometimes darkening of the skin.
The condition is diagnosed through stimulation testing. If the adrenal glands fail to raise cortisol levels appropriately after being given synthetic ACTH, with a rise of at least 7 mcg/dL to a peak of 20 mcg/dL or more, adrenal insufficiency is confirmed. An adrenal crisis, in which cortisol drops dangerously low during illness or physical stress, is a medical emergency that can cause shock and organ failure.
“Adrenal Fatigue” Is Not a Medical Diagnosis
The idea that chronic stress gradually wears out your adrenal glands, causing a vague syndrome of fatigue, brain fog, and poor sleep, has gained traction in popular health media under the label “adrenal fatigue.” A systematic review of the available evidence found no substantiation that this is a real medical condition. No endocrinology society recognizes it as a diagnosis. The symptoms attributed to adrenal fatigue overlap with a long list of other conditions, including sleep apnea, thyroid dysfunction, depression, overwork, autoimmune diseases, and actual adrenal insufficiency, all of which have established diagnostic criteria and treatments. If you’re experiencing persistent fatigue and suspect a cortisol problem, the distinction matters: true adrenal insufficiency is measurable, treatable, and potentially life-threatening if missed.