Why do symptoms get worse at night like fever or cough?

Increased vagal tone: That's rather physiological, parasympathetic part of the nervous system tends to have increased activity at night, the vagus nerve, one of the most complex cranial nerves, is part of that system, it innervates several systems, hence the increased symptoms at night, also the fever is controlled centrally in the brain.This is an intricate complex system cannot be fully understood / explained here