Modern day stress is more closely tied to cognitive processes rather than biological ones. In fact, most stress is caused by a conflict or flaw in conscious reasoning.
Cognition includes both perception and thought. Stress influences thoughts directly and perception indirectly. At first, the mental focus is stronger, but through prolonged stress thoughts tend to lose depth and scatter.
This loss of mental focus influences the state of consciousness, and through it the perceptive process also suffers: we don't notice things we usually should have and we pay attention to things of no consequence.
Cognitive stress disturbs the connection between the subconscious and the conscious mind. In a relaxed, healthy state the subconscious can do the bulk of the work, while the conscious mind can focus on the cherry-picked info and make good, informed decisions. With stress, the conscious mind loses ready access, becomes panicky, and makes rushed and inadequate decisions - which generally worsens the situation in a downward spiral.
The conscious mind has to quiet down at times to allow the subconscious to do its job properly.