Physical exertion releases the hormones endorphin and enkephalins, natural substances in the body that are similar to opium. The brain responds to exercise the same way it would to opium. Like any drug, endorphins and enkephalins are addictive. But the body also builds up resistance. They actually have to seek adventure to satisfy their minds, George Bubenik said.
Many thrill seekers have different sensory receptors in their brains.
The lack of stimulation in the brain is called reward deficiency syndrome. They crave higher levels of the hormones adrenaline, dopamine and serotonin. They do not feel happy unless they are doing something risky. They more and more dangerous stuff they do, the more pleasure they get. Yawning is the body's way of releasing extra carbon dioxide.