Stress
A little bit of stress isn't always a bad thing; it mobilises our bodies and energises us during the coping process. But being overstressed may result in a range of health problems.
The most common perception of stress is the 'fight or flight' scenario, and the story goes something like this. A caveman is confronted by an angry mastodon: he either needs to club it to death or run for his life. A quick surge in the 'stress-hormone' adrenaline allows him to act by strengthening his heartbeat, widening his airways and redirecting blood to his muscles.
But there are two problems with this Neanderthal model. First, the riskiest animals our predecessors had to confront were rabbits and deer, not elephants and tigers. In fact, our hairy forefathers spent most of their time collecting berries and roots with their children, aunties and pals. Just like us, the caveman never stood alone in front of wild animals - unless there had been some kind of terrible mistake.
The second error is to try to relate stress solely to adrenaline. Although the physical effects of frights and acute (short-term) stress are caused by adrenaline, this hormone doesn't enter the brain. Longer-term stress relates to a range of other hormones and brain neurotransmitters.