A few years ago, I attended a workshop on weightlifting to gain continuing education units to keep my certifications up to date. At one point during the workshop, the instructor insisted that there was no such thing as overtraining. I was disappointed to hear this, granted that at that time I was knee deep in my doctoral research in physiology and everything I had learned up until that point had taught me that it is absolutely possible to overtrain your body.
As I reflect back on this, what I realize is that there seems to be a lot of confusion on the subject. From a scientific perspective, I can tell you that YES, if you challenge your body with enough physical stress without a combination of proper recovery and nutrition, you will hit your wall. And no, you don't have to be star athlete or work out for hours a day to be "overtrained". Overtraining simply means you are putting too much demand on your body without letting it catch up with recovery. The "more is better" approach to fitness is NOT a good approach. Your body will want to punch you in the face if you continually push it on top of needing recovery. How do I know this? Because 1) SCIENCE and 2) I've done it to myself and it is NOT FUN.
Chronic overtraining on top of fatigue does a lot of things to your body that will totally derail the awesome goals you set for yourself. It can raises cortisol levels significantly. Cortisol is a hormone that is necessary for survival, but not meant to be circulating at high levels chronically. Having elevated levels of cortisol for an extended period of time actually promotes fat accumulation, decreases immunity, and also leads to muscle breakdown. No dice when you're trying to bust your butt in the gym everyday.
It also increases risk of injury. If you are tired and sluggish during your workouts, guess what goes down the toilet? Your form. Guess what keeps your joints, bones, tendons, ligaments, and muscles safe during workouts? Your form. If you are constantly exhausted and sore during workouts, take as much time as you need from the gym so that you can hit it hard again after you are rested..and remember, overtraining IS real.