The first phase of stress is called the alarm phase, which is ruled by the hormone called adrenaline which activates the flight-or-fight response. This phase lasts for a very brief time and can even go unnoticed by many people. Adrenaline gives you an extra boost through a series of physiological changes to either run from or fight the stressor. You may have noticed increased heartbeat and accelerated breathing just before the important presentation – this is alarm phase and adrenaline rush.
At this moment, only vital body processes receive energy to survive, with the rest being redirected to run from or fight stress. This means many processes in your body get suspended or operate at “low capacity” (among others sexual drive, digestive system, and others).