How it fits together · lesson 2 of 10
How the stress response works
Meet the HPA axis: the hormonal cascade behind fight-or-flight.
6 min read · reviewed October 2026
Two alarms, fast and slow
When your brain perceives a threat — physical or psychological — it fires two systems: a fast nerve-driven surge of adrenaline, and a slower hormonal cascade called the HPA axis that releases cortisol. Together they ready you for action: heart rate up, fuel released, focus narrowed.
The HPA axis cascade
Perceives the threat and releases CRH, kicking off the cascade.
Responds to CRH by releasing ACTH into the bloodstream.
Release cortisol (and adrenaline) — raising blood sugar, heart rate, and alertness.
Cortisol signals back to the brain to shut the response off. Chronic stress blunts this brake, so cortisol stays elevated.
Cortisol is supposed to spike — then fall
A healthy rhythm is cortisol high in the morning, tapering through the day. Chronic stress flattens and dysregulates that curve, which is part of why it disrupts sleep, mood, and metabolism.
What is cortisol's normal healthy pattern across a day?
- Flat all day
- High in the morning, tapering down by night
- Highest at night
- It should always stay near zero
Show the answer →
B.High in the morning, tapering down by night
Cortisol should peak in the morning (the cortisol awakening response) and decline through the day. Chronic stress flattens and dysregulates this rhythm.