Moderate evidence

True overtraining syndrome is uncommon and takes weeks to months of recovery; short-term overreaching is far more usual.

In practice

Feeling flat for a week is not overtraining. It's usually accumulated fatigue, poor sleep, or under-eating, and an easier week fixes it. Persistent, unexplained performance loss lasting months with mood and sleep disturbance is a different matter and warrants medical input.

What it doesn't say

There is no diagnostic test — the consensus definition is one of exclusion, which is exactly why the label gets over-applied.

Source

• Meeusen R, Duclos M, Foster C, Fry A, Gleeson M, Nieman D, Raglin J, Rietjens G, Steinacker J, Urhausen A (2013). Prevention, diagnosis, and treatment of the overtraining syndrome: joint consensus statement of the European College of Sport Science and the American College of Sports Medicine. Medicine & Science in Sports & Exercise, 45(1):186–205. doi.org/10.1249/MSS.0b013e318279a10a