Physiological stress is body response to upset homeostasis of it. Basically, body is self regulating system, and it always seeks the way of returning to "ideal" state, if something pushes it out of its parameters. That happens constantly, but we notice stress only if our body goes out of parameters by large margin, and it responds strongly enough. Shaking in cold, sweating in heat, having high heart rate while exercising and so on is a stress.
Psychological stress is similar in someway, except it is not response to disturbed homeostasis of body, but to disturbed state of mind. I.e., if we have lots of work and we are not sure we will complete it before deadline, we begin feeling pressure, anxiety and so on. Such response of our subconsciousness is kinda supposed to provoke some actions (like working harder to meet deadline) from our consciousness, and return mind closer to peace ("ideal" state).
Stress is tiring, because body/mind has always work harder in stress. This is not always a bad thing, as manageable short time stress can help with training/adaptation, however a long term stress or too strong stress can be very damaging.
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