Key takeaways
Overreaching that recovers in days to a couple of weeks is a normal part of training, while overtraining syndrome can take months to resolve
The core signs are a drop in performance that persists despite rest, plus mood disturbance, fatigue and poor sleep
No single marker diagnoses overtraining, and low energy availability (REDs) must be ruled out because the symptoms overlap
Getting fitter needs overload: you train harder than you are used to, get tired, recover and come back stronger. The problem starts when the recovery part never arrives. The 2013 joint consensus statement of the European College of Sport Science and the American College of Sports Medicine, led by Romain Meeusen, is still the reference document for how to recognise and prevent it.
Functional vs non-functional overreaching vs overtraining syndrome
The consensus describes a continuum, separated mainly by how long it takes performance to come back:
| Stage | What happens | Recovery time | Outcome |
|---|---|---|---|
| Acute fatigue | Normal tiredness after hard sessions | Hours to a few days | Adaptation |
| Functional overreaching (FOR) | Short-term performance drop after an intended hard block | Days to about 2 weeks | Supercompensation: performance improves |
| Non-functional overreaching (NFOR) | Performance stagnates or falls; mood and hormonal changes appear | Weeks to months | Full recovery with rest, but no gain |
| Overtraining syndrome (OTS) | Prolonged maladaptation; underperformance persists despite rest | Months, possibly years | Diagnosed by exclusion |
The authors stress that NFOR and OTS show the same clinical, hormonal and other signs, and that OTS can often only be diagnosed in hindsight, once the time course is clear. Confounders such as poor nutrition, infections, work or family stress and sleep problems are common at the NFOR stage.
Overtraining symptoms
The defining feature is a sport-specific drop in performance together with mood disturbance that does not resolve after several weeks of recovery. Commonly reported symptoms include:
- Slower times or lower power at the same perceived effort, or the same pace feeling much harder
- Persistent fatigue and heavy legs, even after easy days
- Lower motivation, irritability, low mood, loss of vigour
- Poor or unrefreshing sleep
- Frequent colds or upper respiratory infections
- Loss of appetite or unwanted weight change
- Niggles and injuries that keep piling up
Mood questionnaires track this well: athletes in heavy training consistently report more tension, fatigue and depression scores and less vigour, which reverses during a taper. If low mood persists beyond training, take it seriously in its own right (VO2max, mental health and depression).
Overtraining HRV and resting heart rate
Many people hope their watch will spot overtraining. The evidence is more nuanced:
- Resting heart rate. The consensus notes that changes in resting heart rate are not consistently found in OTS. It cites a meta-analysis in which short overload periods (under two weeks) raised resting heart rate by about 4.5 bpm and lowered maximal heart rate by about 7.5 bpm; after longer intensified training, only the lower maximal heart rate remained (about 3.6 bpm).
- HRV. Studies show no change, inconsistent changes or shifts in parasympathetic activity. The consensus calls HRV "a tool in theory" that does not yet give consistent results.
- The paradox. In a study of 21 triathletes, Le Meur and colleagues found that functional overreaching after three weeks of overload (performance down 9%) was linked with higher, not lower, parasympathetic HRV indices and a lower resting heart rate. Once-weekly measurements missed it; only daily readings averaged over each week detected the change, which reversed during the taper.
- Plews and colleagues described a similar issue in elite athletes: HRV can fall despite a lower resting heart rate ("saturation"), and both rises and falls have been linked with poor adaptation. They recommend rolling averages and learning each athlete's individual pattern.
The practical takeaway: a single low HRV morning means little. A trend over 7 days that moves well outside your normal range, together with worse performance and mood, means a lot more. See resting heart rate, HRV and recovery for how to read the numbers.
Overtraining vs REDs: the energy overlap
The consensus explicitly lists dietary calorie restriction, low carbohydrate or protein intake and iron deficiency among the things to rule out before diagnosing OTS. That overlap is now captured by Relative Energy Deficiency in Sport (REDs).
The 2023 IOC consensus statement on REDs, led by Margo Mountjoy, describes a syndrome of health and performance problems in women and men caused by low energy availability: too little energy intake for the energy spent in exercise. It draws on more than 170 new studies since 2018, highlights the role of low carbohydrate availability and mental health, adds more data on men, and introduces the REDs Clinical Assessment Tool version 2 for diagnosis and risk stratification.
| Feature | Overtraining syndrome | REDs |
|---|---|---|
| Root cause | Training plus non-training stress exceeding recovery | Low energy availability |
| Key symptoms | Underperformance, fatigue, mood disturbance | Similar, plus bone, menstrual, hormonal and metabolic effects |
| Who | Mostly endurance athletes in heavy training | Female and male athletes, any sport |
| First step | Rest and rule out other causes | Assess intake vs expenditure; restore energy |
If you are losing weight unintentionally, have missed periods, or have had a stress fracture, consider REDs first. See VO2max, women, the menstrual cycle and menopause and blood tests for athletes for useful checks such as ferritin.
How to prevent overtraining
The consensus recommends keeping good records of performance, training load, mood and sleep, and adjusting quickly. Practical rules:
- Build in recovery. Easy weeks every three to five weeks (periodisation for endurance).
- Keep easy days easy. Most volume below the first threshold (80/20 polarised training).
- Eat enough, particularly carbohydrate around hard sessions.
- Don't train through illness (detraining: how fast you lose VO2max).
- Watch for life stress. Work, travel and poor sleep add to the load your body has to absorb.
Overload before a race works best when you are tired but not overreached. In a study of triathletes, those who became functionally overreached gained less from the taper and had more infections (tapering before a race).
When to see a doctor: performance and mood that stay down after two to three weeks of reduced training, unexplained weight loss, chest pain, fainting or palpitations. Organic disease, infection and nutritional deficiencies need to be excluded.
Frequently asked questions
What are the first signs of overtraining?
Usually a drop in performance at the same effort, heavy legs that do not recover after easy days, worse sleep, irritability or low motivation, and frequent colds. A resting heart rate that stays above your normal for several days is another clue.
How long does it take to recover from overtraining?
Functional overreaching recovers within days to about two weeks. Non-functional overreaching takes weeks to months, and overtraining syndrome can take months or longer, according to the ECSS and ACSM consensus.
Can your watch detect overtraining?
Not reliably on its own. HRV and resting heart rate trends can flag accumulated fatigue, but they change in both directions with overload and vary a lot day to day. Use weekly averages together with performance and mood.
What is the difference between overtraining and REDs?
Overtraining syndrome is maladaptation to training stress with no other explanation, while REDs is caused by eating too little for the training you do. The symptoms overlap heavily, so low energy availability has to be ruled out before calling it overtraining.
Sources
- Meeusen R, Duclos M, Foster C, et al. 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. Med Sci Sports Exerc, 2013. PubMed
- Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med, 2023. PubMed
- Le Meur Y, Pichon A, Schaal K, et al. Evidence of parasympathetic hyperactivity in functionally overreached athletes. Med Sci Sports Exerc, 2013. PubMed
- Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports Med, 2013. PubMed
- Aubry A, Hausswirth C, Louis J, Coutts AJ, Le Meur Y. Functional overreaching: the key to peak performance during the taper? Med Sci Sports Exerc, 2014. PubMed