Cortisol is the body’s daytime accelerator. Released by the adrenal glands under direction of the brain’s stress system (the HPA axis), it mobilizes fuel, keeps blood pressure responsive, tempers inflammation, sharpens attention, and helps set the sleep–wake rhythm. It peaks within 30–60 minutes after waking, then steadily declines to very low levels near midnight. Most labs define a broad morning reference band; in healthy adults, values sit near the middle of that morning range, with clearly low late‑night levels and a steep daytime slope.When cortisol is lower than expected for the time of day—or the curve is flat—energy delivery and vascular tone sag. People often feel profound fatigue, dizziness on standing, brain fog, nausea, salt craving, and low mood; blood pressure, sodium, and glucose can run low. Primary adrenal failure can add skin darkening, while pituitary causes lack this. Children may have poor growth or recurrent hypoglycemia. Women can notice cycle irregularity. After pregnancy, values gradually reset toward nonpregnant ranges.When cortisol runs high—or stays elevated at night—the body shifts toward storage and wear‑and‑tear. Central weight gain, high blood pressure, high glucose, thin skin with easy bruising, bone loss, infections, anxiety or irritability, and poor sleep can emerge. Women may have irregular periods or hirsutism; men may note reduced libido. In children, slowed linear growth despite weight gain is a red flag. Pregnancy raises total cortisol physiologically, so ranges shift upward.Big picture: Cortisol connects brain, adrenals, metabolism, immunity, bone, and cardiovascular systems. Persistent dysregulation—too much, too little, or a blunted rhythm—tracks with diabetes risk, osteoporosis, depression, and heart disease, underscoring why timing, pattern, and context matter.
