Final answer:
Sharon's 'runner's high' is likely caused by the release of endorphins, which are natural chemicals in the brain that can induce euphoria and pain relief.
Step-by-step explanation:
Sharon is experiencing a natural "high" during her long-distance running due to the release of chemicals in her brain known as endorphins. Endorphins are often released during vigorous aerobic exercise and contribute to the feeling of euphoria and pain reduction, commonly described as a "runner's high." Unlike adrenaline, which is produced in stressful or exciting situations, or stimulants, which are artificial substances, endorphins act as natural painkillers and mood elevators.