It is called the fight-or-flight response, and it happens when we are put in a situation where our body needs to react accordingly. Adrenaline is dumped into our bodies, prepares us to either stay and challenge the situation head on or get out of Dodge and live to fight another day
The most natural cause of an adrenaline rush is when one finds himself in a precarious or even life-threatening situation, which is the adrenaline was made. It prepares the body by quickening pulse and add strength (sometimes experienced as almost superhuman) and speed to our common capabilities. Thus, an individual fight as best or run away as fast as possible.
While a random regime of exercise is unlikely to cause an adrenaline rush, longer, more intense periods of exercise triggers the phenomenon. This can be a caused by weightlifting, running or other high-intensity exercise where the body is pushed beyond their usual boundaries. The adrenaline can help the individual to reach new plateaus in exercise their and stretch the limits of his fitness level.
Certain drugs, most of which are stimulants, can lead to an excess production of adrenaline. Drugs such as cocaine, PCP and even caffeine can trigger a rush, which contributes to the overall high individual feel. Usually it takes more often the individual substance, the less potent the effect be on the body, including the adrenal glands.
Although very tired can bring on an adrenaline rush, as the drug acts as a pick-me-up, allowing a person to work harder and longer than they would otherwise be able to.
For those who are particularly sensitive to sugar (especially children), overuse of sweetener can cause excessive adrenaline production. This is usually a bad thing, as usual adrenaline production will not only weaken the glands, but crashes after the fact will leave the individual feeling weak and lethargic.