Your autonomic nervous system runs everything you do not consciously steer: your heart rate, your breathing, your digestion and your recovery. It has two branches that work against each other. The sympathetic branch is your accelerator, the parasympathetic branch your brake.
Both should be working, just not at once.
What I see most often as a training partner is an athlete with a solid programme and an accelerator that never gets released. Hard sessions, busy work days, late meals, short nights. The body never reaches the brake, and you see it in an HRV that sits below its own normal for months.
What is the autonomic nervous system?
It is the part of your nervous system that runs your organs without a conscious command. You decide whether to lift your hand, but not how fast your heart beats or how hard your stomach works. This system handles that, day and night, including while you sleep.
It has two branches. The sympathetic branch prepares you for action, the parasympathetic branch for rest, digestion and repair. The Dutch Hartstichting describes that same interplay as the reason your heart rate rises during effort and falls at rest.
The seesaw image helps. When one side goes up, the other comes down. Healthy does not mean the accelerator is off, but that the seesaw moves with whatever you are doing.
What does the sympathetic nervous system do?
It prepares you for effort and danger. Your heart rate rises, your breathing quickens, blood moves to your muscles and your pupils widen. Digestion drops down the priority list, because digesting can wait until after the sprint.
That is exactly what you want during a heavy set.
The problem starts when that system stays switched on outside training. Chronic work pressure, sleep loss and stacking training load keep your accelerator pressed while there is nothing to run from. Your HRV then falls, including on days you do nothing.
In endurance athletes that can build into the picture we call overtraining, where heart rate measures move with your training status (PMID 26888648).
What does the parasympathetic nervous system do?
It puts your body into recovery mode. Your heart rate falls, your breathing settles, digestion starts up and your body repairs tissue. This branch runs largely through the vagus nerve, the longest nerve in your body.
This is the state you get stronger in, not during the session itself.
The variation between your heartbeats comes mostly from this side. That is why the HRV metric RMSSD tracks parasympathetic activity so closely (PMID 29034226). When your brake works harder, your beat-to-beat pattern gets less regular, and that is precisely what you measure.
How does that balance show up in your HRV?
As a shift in your overnight average. If your accelerator dominates, your HRV falls and your resting heart rate rises. If your brake gets more room, the opposite happens. The useful part is that you can see the same shift in your blood.
This table is how I keep the four states apart.
| State | What you notice | What your HRV does | What you can check in blood |
|---|---|---|---|
| Balance | Fit, good sleep, hunger at normal times | Around your own average | Nothing needed |
| Acute stimulus after training | Heavy legs, 1 to 2 days | 1 to 2 nights lower | CK if soreness is extreme |
| Stacking load | Poor sleep onset, short fuse, flat feeling | Weeks below your normal | Cortisol, hs-CRP |
| Underlying cause | Tired without explanation, cold, breathless | Low and stays low | Ferritin, TSH, free T4, glucose |
Picture two athletes who both sit at 41 ms for a week. One has just finished a heavy training week, the other has trained easy for a month and sleeps well. Same number, and only the second one has something to investigate.
Can you train your parasympathetic system?
To a degree, yes. The best studied method is breathing slowly at around six breaths per minute, described in the literature as HRV biofeedback. During and shortly after that exercise, HRV rises measurably, and the effect has been replicated repeatedly (PMID 25101026).
Do not expect a miracle from it.
What pays off structurally is duller: sleeping enough, building your aerobic base, limiting alcohol and planning real rest days. A cold shower or a sauna session shifts your balance temporarily, but does not move your average by itself. What those habits actually do sits in sauna and cold exposure.
My order is simple: sleep first, then load, then breathing technique. Reverse that order and you are practising five minutes a day against a two-hour sleep debt.
Which blood values go with a disturbed balance?
No blood value measures your autonomic nervous system directly. Autonomic imbalance is, however, associated with a set of markers you can simply have drawn, including inflammatory activity and factors linked to cardiovascular risk (PMID 19910061).
In practice I look at four things. Cortisol moves with your total load. hs-CRP shows silent inflammation. Ferritin shows your iron stores, and TSH tells you whether your thyroid is driving your heart rate.
That combination sits in InsideTracker, so you can place your recovery data next to real values. If you only want to know what stress is doing, the cortisol test is the smaller starting point.
Do one thing this week: look back over your last fourteen nights and count how many days your accelerator was genuinely released. Below four, your programme is not the problem.
Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
References
- Thayer JF, et al. The relationship of autonomic imbalance, heart rate variability and cardiovascular disease risk factors. International Journal of Cardiology, 2010. PMID 19910061.
- Lehrer PM, Gevirtz R. Heart rate variability biofeedback: how and why does it work? Frontiers in Psychology, 2014. PMID 25101026.
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Frontiers in Public Health, 2017. PMID 29034226.
- Bellenger CR, et al. Monitoring athletic training status through autonomic heart rate regulation: a systematic review and meta-analysis. Sports Medicine, 2016. PMID 26888648.
- Hartstichting. Heart rate and heart rhythm.
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