Overtraining syndromeOTS · Unexplained underperformance syndrome
A prolonged performance decline caused by training load outpacing recovery.
Overtraining syndrome is a state of long term performance decline with fatigue, disturbed sleep, mood changes and loss of motivation, which persists despite adequate rest. It is the far end of a spectrum and it is genuinely uncommon.
The spectrum matters more than the label. Functional overreaching is a short term dip after a deliberately hard block, from which performance rebounds above baseline within days to a couple of weeks, and it is a normal part of training. Non-functional overreaching takes weeks to months to resolve. Overtraining syndrome takes months and sometimes longer.
Almost everyone who says they are overtrained is under-recovered, which is a different and much better problem. Insufficient sleep, insufficient food, unrelenting life stress and no planned easier weeks produce most of the symptoms, and they respond to addressing those inputs.
Diagnosis is by exclusion, which is the awkward part. There is no test for it, and the presentation overlaps with anaemia, thyroid disease, depression, inadequate energy availability and infections including glandular fever, all of which need ruling out before the label is applied.
Relative energy deficiency in sport belongs in the same conversation, since chronically eating too little for the training being done produces overlapping symptoms and has its own consensus framework and consequences for bone and hormonal health.
The monitoring tools people reach for, resting heart rate, heart rate variability, session ratings, subjective wellbeing, work best as trends against your own baseline rather than as single readings.
In practice, plan easier weeks before you need them, eat enough for the training you are doing, and take a persistent unexplained decline to a doctor rather than to a longer taper.