Here's a surprising case of cause and (delayed) effect: your feelings of fatigue and irritation today could be the response of your immune system to stress you encountered last week.
Moodiness, tiredness, trouble sleeping--these could be reactions to stress or symptoms of sickness. The ability to fight off illness evolved long before the fight-or-flight stress response, say University of Colorado psychologists Steven Maier, Ph.D., and Linda Watkins, Ph.D. That came along later, co-opting the biochemical routes used by the immune system.
What may be efficient from an evolutionary point of view is a problem for modern folk for whom stress is a life fixture. Whether it's a 24-hour bug or a looming deadline, our bodies react by s-l-o-w-i-n-g down.
When fighting an infection, this sluggishness allows available energy to drive up body temperature which in turn kills the virus. When dealing with stress, the slowdown allows the body to save up its energy store in case the initial stress is a warning of more danger to come.
