As for what fear is biologically speaking, when a person experiences fear, certain areas in their brain such as the amygdala and the hypothalamus are immediately activated and appear to control the first physical response to fear.
Rapid heart rate, increased blood pressure, tightening of muscles, sharpened or redirected senses that can lead us from danger, dilation of the pupils (to let in more light), increased sweating.
Fear is food for the demonic realm, it works like Faith only in reverse. It becomes UNADAPTIVE in your life once you become focused on your fears rather than on what your fear is warning you about, thus you are not able to learn the lesson it teaches you.
This is why fear can be irrational as common sense, and rational thought are POST FEAR REACTIONS. It is impossible to obey God out of this form of fear, it debilitates our faith and trust in him causing us not to receive what he wants us to have.
Too Fear our God is too give him his due, he is infinitely more powerful than all of the energy found in the universe combined, he alone holds every Atom in the universe together keeping you and me from floating into space. The same God in Christ that died as a man and rose from death to free us all from sin.
This is the God we are to stand in Awe of everyday, a God bigger than the Universe who felt the need to live in my heart, if that doesn't cause pause in your life...YOUR PROBABLY DEAD!