I'm going through a rather strenuous time right now--one consequence of doing your job a bit too well is that you suddenly get heaped with tons of work. Added to this is a four-hour commute and my usual life and it's no wonder I've given up on trying to get the recommended 8 hours of sleep (I might almost get there on the weekends).
Stress, strictly speaking, is anything from the outside world that affects you. It is not, by definition, a bad thing: if tomorrow you won the lottery, that too is stress.
In the days of yore, when we hadn't yet earned the genus Homo, stress was running from a jaguar--and fighting like hell when it caught you. The release of epinephrine and norepinephrine enables the muscles to work much harder. Pain responses get blunted. Blood is shunted from organs that don't need it (your gut) to those that do at this moment, as your entire body is devoted to one thing--getting the hell out of there, or beating the crap out of whatever's trying to beat the crap out of you.
Nowadays, we have a more evolved brain, and far more involved lives. Social interactions are no longer limited to who gets pickings at the termite mound. Being able to think means that what we make of a scenario can greatly affect our body's response to it. Someone who has a breakdown on I-95 can panic, get angry, rant at an angry God, or call AAA. These days, very few of us will ever encounter stress of the sort our ancestors did, unless you're unfortunate enough to fall into a den of lions. But the evolutionary mechanisms by which we deal with stress--open the floodgates for epinephrine--have remained the same.
Is this a good thing, or not? Stay tuned.