One of the forms of stress is based on external factors affecting your comfort zone as described on citation below.
“Stress is a response to pressure or threat. Under stress we may feel tense, nervous, or on edge. The stress response is physical, too. Stress triggers a surge of a hormone called adrenaline that temporarily affects the nervous system. As a result, when you’re nervous or stressed you might feel your heartbeat or breathing get faster, your palms get sweaty, or your knees get shaky.” cited from http://kidshealth.org/en/teens/stress.html
Generally stress is anything forcing you to use more energy to compensate it. So you may need to write test, talk with somebody you like a lot but you are afraid of rejections, do a physical intensive work or e.g. have infections. It can be intellectual or physical.
Stress could be a good thing or bad too. It can stimulate you to more efficient work or push you down.
Anyway it is bad stress if it is responsible for fatigue or is increasing your fatigue existing state.
Why it is happen?
During stress your body use more energy to compensate it. In other words it drains energy from your body. You may experience fatigue or increased level of existing one if you have not enough energy to compensate it and do all daily tasks.
Fatigue is a physical or mental weariness resulting from exertion. In this case you use more energy you should and this lead to fatigue. Also this energy can be drain by stress or intellectual work, not only by physical exercise, physical work etc.
Converting it on human language – your body is begging for more energy. You should have a decent rest to restore energy buffer.
More about chronic fatigue syndrome is here >>> http://chronic-fatigue-syndrome.microalpha.com
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