You could look at this in a different way. The psychological phenomena of the contagiousness of yawning is an adaptation that evolved in groups of animals to help in alerting them to danger. When one sees someone yawn, you yawn as well. The brain initiates this behavior, as the yawn itself draws an increased amount of oxygen into the lungs, sending more to the brain, and aids in "waking" individuals (which is also why we yawn when awake) to help in completing a task (such as fleeing danger).

Yawning requires calories but can make one feel energized, so that something like the act of exercise CAN in reality make one LESS tired or energized by the simple fact it initiates the burning of fuel in the body. Some of this fuel that is being burned is not even readily available to the rest of the body under normal circumstances, so you're not necessarily even depleting carbs and what not in the amounts that would lead to immediate fatigue. At any rate, one in theory exercises, in a manner of speaking, by being awake, period. You certainly can feel more energized in the mid afternoon as opposed to the waking hours of the day.

At any rate, that exercise IS the burning of FUEL, can explain in some situations why you might feel energized, with or without the release of endorphins. People can feel energized from light exercise that certainly does not release endorphins, so that alone could not be the case.