Over training is a Physical & Emotional condition that occurs when an individuals exercise volume totally exceeds their recovery rate or capacity.
So why do people overtrain? Most of us believe More is Better. Others have an addictive personality & get hooked very easily because of the Endorphin rush which is related to exercise. Endorphins is a chemical released by the brain when one exercises. Hence one has that "Feel Good" feeling.
Over training is very common amongst people who lift weights. Runner's also easily fall into this overtraining category because they never feel they've done enough distance before a marathon. So they end up doing too much mileage.
Improvements in strength and fitness occur only after the rest period following hard training. This process may take a number of days to complete, depending on the Intensity & duration of exercise which may lead to and overtrained state.
Then there is the Physiological effects of overtraining: Decreased muscle strength, Chronic muscle soreness, Depressed immune system which leads to constant infections, flu & colds, Insomnia etc
Take a break from exercise and allow your body to recover properly
Reduce your training intensity
Get lots of sleep
If you training hard, allow rest DAYS in between
Training "Smartly & Wisely" is the way to go. You will reach remarkable goals & results if you only trained Smartly. The question i always ask all my clients..."Are you a professional athlete?" If the answer is No, then don't try & train like one.