Overtraining is a real clinical entity and it is also the most over-claimed condition in amateur endurance sport. Getting the distinction right matters, because the two things it is confused with have completely different fixes.
Three states, not one
Acute fatigue. You did a hard session and you feel it. Resolves in a day or two. This is training working.
Functional overreaching. A block of accumulated hard work has pushed you into a short-term performance dip. Resolves with a few easy days or a deload week, and typically leaves you fitter than before. This is also training working, deliberately.
Overtraining syndrome. Months in the making, months to resolve, usually involving hormonal, immune and mood disruption well beyond training capacity. Genuinely rare outside of athletes training at high volume with insufficient fuelling.
The practical test is time. If a week of genuinely easy training restores you, it was overreaching, and the block did its job.
The measurable signs, in rough order of appearance
1. Resting heart rate drifts upward
The most reliable objective marker available to a civilian. Not one high morning, a sustained drift of several beats above your own baseline across a week or more. It is a better signal than HRV for this particular purpose because it is less volatile night to night.
2. HRV stays down and stops rebounding
HRV should drop after hard work and come back within a day or three. The diagnostic pattern is not the dip, it is the failure to rebound after easy days. Detail on the many other causes of a low reading is in why is my HRV low.
3. Sleep gets worse while you get more tired
Counterintuitive and very characteristic. Difficulty falling asleep, more waking, and non-restorative sleep despite obvious fatigue. If your training load is climbing and your sleep quality is falling, that combination deserves attention on its own.
4. The same session feels harder
Elevated perceived exertion at unchanged pace and heart rate. Often the first thing an athlete actually notices, and it usually precedes any measurable performance drop.
5. Mood, motivation and appetite change
Irritability, flat mood, and a reluctance to train that is unlike your normal. These are not soft signs. In the research literature, mood disturbance is among the earliest and most consistent markers.
6. Performance finally drops
By the time times slow or power falls at a given heart rate, the process has been running for a while. This is a late sign, not an early one.
What to do about it
For overreaching, the answer is unglamorous: reduce volume by roughly half for a week, keep some intensity but cut the duration, sleep as much as you can, and eat enough. Then reassess. Most people find their resting heart rate settles within a few days.
The thing not to do is add more recovery modalities on top of the same load. Ice baths, massage guns and compression do not address a load that is too high for your current capacity.
For anything that survives two weeks of genuine rest, stop self-diagnosing and see a doctor. Nothing on this page is medical advice and this is precisely the point where an app stops being the right tool.
For how to structure the load so this does not happen, see building a split around muscle recovery and how many rest days you actually need.