When you exercise, your body releases chemicals called endorphins. These endorphins interact with the receptors in your brain that reduce your perception of pain.

Endorphins also trigger a positive feeling in the body, similar to that of morphine. For example, the feeling that follows a run or workout is often described as “euphoric.” That feeling, known as a “runner’s high,” can be accompanied by a positive and energizing outlook on life.

Endorphins act as analgesics, which means they diminish the perception of pain. They also act as sedatives. They are manufactured in your brain, spinal cord, and many other parts of your body and are released in response to brain chemicals called neurotransmitters. The neuron receptors endorphins bind to are the same ones that bind some pain medicines. However, unlike with morphine, the activation of these receptors by the body’s endorphins does not lead to addiction or dependence.

Endorphins are long chains of amino acids, or polypeptides, that are able to bind to the neuroreceptors in the brain and are capable of relieving pain in a manner similar to that of morphine. There are three major types of endorphins: beta-endorphins are found almost entirely in the pituitary gland, while enkephalins and dynorphins are both distributed throughout the nervous system.

Scientists had suspected that analgesic opiates, such as morphine and heroin, worked effectively against pain because the body had receptors that were activated by such drugs. They reasoned that these receptors probably existed because the body itself had natural painkilling compounds that also bonded to those receptors.

When scientists in the 1970s isolated a biochemical from a pituitary gland hormone that showed analgesic properties, Choh Li, a chemist from Berkeley, California, named it endorphin, meaning “the morphine within.” Besides behaving as a pain reducer, endorphins are also thought to be connected to euphoric feelings, appetite modulation, and the release of sex hormones.

Prolonged, continuous exercise contributes to an increased production of endorphins and, in some people, the subsequent “runner’s high.”