Overtraining is a long-term unexplained decrease in performance, unlike for, e.g., feeling tired after a race or hard session for several+ days, which is generally referred to as overreaching and is an important and needed aspect of periodised training.<br /><br />Currently, there's no markers of overtraining that can be identified within a laboratory. Scientists have tried to find markers by doing mood checks, blood bio-chemistry, and HR etc.<br /><br />Recovery from overtraining can take from several week, to months to year+. if you recover within a few days, it was overreaching!<br /><br />some studies have looked at massive increases in training loads (e.g. more than double your normal training amount) and overtraining hasn't been induced, so it may well also be caused by other stressors in addition, such as family, school, and work problems.<br /><br />I *think* i've also seen the problem referred to in conjunction with chronic glycogen depletion -- which means that people aren't eating enough carbos to support their training loads.<br /><br />There's also research suggesting two types of overtraining, where in one type, resting HR increases, and in the other resting HR decreases, so you have to be aware of changes in both direction.<br /><br />Perhaps, the best advice is to gradually increase your training, eat well, and have recovery weeks scheduled on a regular basis. Having a coach should help to, as they'll be able to objectively review the training logs you keep.<br /><br />Ric