Allostasis is an expanded view of homeostasis, which occurs when the body is physically balanced. This definition is important for understanding the concepts of stressors and stress-response: Stressors remove the body from allostasis, and the stress-response reestablishes allostasis.

Analgesia refers to an inability to sense pain. Analgesia occurs through a rise in opioid or opiate chemicals. It can occur as a natural phenomenon, such as when a wounded soldier does not feel their injury or, more commonly, when someone runs long enough to experience a “runner’s high.” Analgesia can also arise from artificial circumstances, such as through opiate medications or acupuncture, which trigger a release of natural opioids. Hyperalgesia occurs when the emotional perception of pain is intensified, such as when a person with a fear of needles is hypersensitive to vaccinations.

Anxiety disorders are a group of mental health disorders associated with fear, vigilance, and insecurity. Subtypes of anxiety include generalized anxiety disorder, phobias, obsessive-compulsive disorder, and post-traumatic stress disorder. Symptoms of anxiety may include panic attacks, hyperventilation, ritualized behavior, and vigilance.