Picture a deer in a field. A twig snaps. In a flash, its body gets ready to run. You have that same fast alarm inside you. It switches on when something feels like a threat.
fight-or-flight
Your body's fast alarm. It gets you ready to fight a threat or run from it.
A scientist named Walter Cannon first named this alarm. He called it the fight-or-flight response. The alarm has not changed for thousands of years.
Sources
Harvard Health, "Understanding the stress response" — the fight-or-flight cascade
Roelofs, "Freeze for action," PMC5332864
Cleveland Clinic, "HPA Axis: The Stress Response System"
Bruce McEwen, "Allostasis and Allostatic Load," Nature
Herbert Benson, The Relaxation Response (1975)
Starkman et al., "Decrease in cortisol reverses human hippocampal atrophy following treatment of Cushing's disease," Biological Psychiatry, 1999
Gilbertson et al., "Smaller hippocampal volume predicts pathologic vulnerability to psychological trauma," Nature Neuroscience, 2002
Robert M. Sapolsky, Why Zebras Don't Get Ulcers, 3rd ed., Holt Paperbacks, 2004 — https://us.macmillan.com/books/9781429935654/whyzebrasdontgetulcers/
Mark A. Uphill, Claire J. L. Rossato, Jon Swain & Jamie O'Driscoll, "Challenge and Threat: A Critical Review of the Literature and an Alternative Conceptualization," Frontiers in Psychology 10:1255, 2019 — https://pmc.ncbi.nlm.nih.gov/articles/PMC6614335/
Marc A. Russo, Danielle M. Santarelli & Dean O'Rourke, "The physiological effects of slow breathing in the healthy human," Breathe 13(4):298-309, 2017 — https://publications.ersnet.org/content/breathe/13/4/298
Michael Meehan & Fred Shaffer, "Do Longer Exhalations Increase HRV During Slow-Paced Breathing?", Applied Psychophysiology and Biofeedback, 2024 — https://pmc.ncbi.nlm.nih.gov/articles/PMC11310264/
Evgeny Vaschillo, Bronya Vaschillo & Paul M. Lehrer, "Characteristics of resonance in heart rate variability stimulated by biofeedback," Applied Psychophysiology and Biofeedback 31(2):129-142, 2006 — https://pubmed.ncbi.nlm.nih.gov/16838124/
Bruce S. McEwen & John H. Morrison, "Stress Effects on Neuronal Structure: Hippocampus, Amygdala, and Prefrontal Cortex," Neuropsychopharmacology 41:3-23, 2016 — https://www.nature.com/articles/npp2015171
Christoph Anacker et al. / Frontiers in Synaptic Neuroscience, "Chronic Stress Effects on Hippocampal Structure and Synaptic Function," 2010 — https://www.frontiersin.org/journals/synaptic-neuroscience/articles/10.3389/fnsyn.2010.00024/full
Jan Born & Steffen Gais, "HPA Axis and Sleep," Endotext, NCBI Bookshelf — https://www.ncbi.nlm.nih.gov/books/NBK279071/
Karin Roelofs, "Freeze for action: neurobiological mechanisms in animal and human freezing," Philosophical Transactions of the Royal Society B 372:20160206, 2017 — https://pmc.ncbi.nlm.nih.gov/articles/PMC5332864/ (already cited; add full author/journal/year)
Cleveland Clinic, "HPA Axis: The Stress Response System" — https://my.clevelandclinic.org/health/body/hypothalamic-pituitary-adrenal-hpa-axis (already cited; the older /health/body/hpa-axis slug now 404s)
Bruce S. McEwen, "Allostasis and allostatic load: implications for neuropsychopharmacology," Neuropsychopharmacology 22(2):108-124, 2000 — https://pubmed.ncbi.nlm.nih.gov/10649824/ (already cited; citation verified exact)
Read end to end and corrected: August 10, 2026. Numbers checked: August 10, 2026.