SDNN
SDNN — the standard deviation of normal-to-normal (NN) heartbeat intervals — is the most widely used measure of heart rate variability (HRV). It shows how much the time between heartbeats changes, and so how flexibly the autonomic nervous system adapts the heart to breathing, posture, stress and rest. Low SDNN is an independent marker of cardiovascular risk.
What SDNN measures
A healthy heart does not beat like a metronome. Even at rest, the interval between two beats (the RR or NN interval) changes by tens of milliseconds from beat to beat. SDNN summarises all these fluctuations in one number: the larger it is, the more variable — and usually the more adaptable — the heart rhythm.
Only normal sinus beats are used ("NN" instead of "RR"): extrasystoles, artefacts and pauses must be removed before calculation, otherwise SDNN can be falsely high.
Physiology
The sinus node fires spontaneously at roughly 100 beats per minute. The autonomic nervous system constantly adjusts this rate:
- The parasympathetic (vagus) nerve slows the heart within one beat via acetylcholine. Its activity rises with each exhalation and falls with each inhalation, producing respiratory sinus arrhythmia — the main source of short-term variability.
- The sympathetic system speeds the heart via noradrenaline more slowly, over several seconds. It contributes to slower oscillations linked to blood-pressure regulation (baroreflex, ~0.1 Hz).
- Thermoregulation, hormones and the sleep–wake cycle create very slow changes visible in 24-hour recordings.
Because SDNN includes all of these components, it is a measure of total variability. In 24-hour recordings it reflects the circadian rhythm as well; in 5-minute recordings at rest it depends mostly on vagal activity and breathing.
How it is calculated
SDNN is the ordinary statistical standard deviation of all NN intervals in the recording:
where NNi is each interval, NN̄ the mean interval and N the number of intervals. Variance grows with the length of the recording, so SDNN values from recordings of different length cannot be compared. International standards recommend either short-term (5-minute) or long-term (24-hour) measurement, each with its own norms.
Reference values
| Recording | Healthy adults | Interpretation |
|---|---|---|
| 24-hour Holter | 141 ± 39 ms (Task Force 1996) | >100 ms normal · 50–100 ms moderately reduced · <50 ms markedly reduced, high risk |
| 5-minute, supine rest | Mean ≈ 50 ms, usual range ≈ 32–93 ms (Nunan 2010) | Compare with your own baseline and age; values below ~20 ms deserve attention |
| Ultra-short (1 min, wearables) | No validated norms | Useful only for following trends in the same person under the same conditions |
Low SDNN and health risks
Reduced HRV means the heart is driven more by sympathetic and less by vagal influence, or that the sinus node responds poorly to nervous signals. This pattern is not a disease in itself, but it accompanies — and often predicts — many conditions.
| Condition | What is known |
|---|---|
| After myocardial infarction | In the classic study by Kleiger et al., patients with 24-hour SDNN <50 ms had about 5.3 times higher mortality than those with >100 ms. Low HRV predicts sudden cardiac death and arrhythmic events. |
| Chronic heart failure | Low SDNN is an independent predictor of death from progressive heart failure (UK-HEART study). |
| Diabetes mellitus | Falling HRV is one of the earliest signs of cardiovascular autonomic neuropathy, often years before symptoms such as dizziness on standing or silent ischaemia. |
| Hypertension, obesity, metabolic syndrome | Associated with sympathetic overactivity and lower HRV. |
| Depression, anxiety, chronic stress, burnout | Consistently associated with lower vagally mediated HRV. |
| Sleep apnoea, sleep deprivation | Reduce HRV and increase night-time sympathetic activity. |
| Acute illness, infection, sepsis | A sudden fall in HRV can precede clinical deterioration. |
| Overtraining in athletes | A sustained fall from the personal baseline suggests insufficient recovery. |
High SDNN
High SDNN is usually a good sign — it is typical of young, physically active people with strong vagal tone. However, a very high value should be checked against the raw ECG:
- Ectopic beats and artefacts (extrasystoles, missed or extra detected beats) inflate SDNN if they are not filtered out.
- Atrial fibrillation makes RR intervals irregular; SDNN is then very high but meaningless as an HRV measure.
- Sinus node dysfunction (sick sinus syndrome) and second-degree AV block can produce long pauses that raise SDNN.
What affects SDNN
- Age — the strongest physiological factor; HRV declines steadily from early adulthood.
- Heart rate — a faster heart rate mathematically and physiologically lowers SDNN.
- Breathing — slow, deep breathing (about 6 breaths per minute) markedly increases short-term SDNN.
- Posture and time of day — values are higher lying down and during the night.
- Recent food, caffeine, alcohol, nicotine, physical exertion — all can lower HRV for hours.
- Medicines — beta-blockers usually raise HRV; anticholinergic drugs, tricyclic antidepressants and sympathomimetics lower it.
For repeatable short-term values, measure at the same time each day (ideally in the morning after waking), lying still for 5 minutes, breathing normally, before coffee and exercise. Follow your trend rather than one single number.
How to improve HRV
- Regular aerobic exercise (brisk walking, cycling, swimming) — one of the most effective ways to raise HRV.
- Enough sleep (7–9 hours) and a regular sleep schedule; treatment of sleep apnoea if present.
- Slow-breathing exercises and HRV biofeedback (around 6 breaths per minute for 10–20 minutes daily).
- Weight loss, limiting alcohol, stopping smoking.
- Good control of blood pressure and blood glucose.
- Stress-management techniques: mindfulness, yoga, psychological support.
Consult a cardiologist if SDNN is persistently low for your age (24-hour <50–70 ms or short-term values that keep falling), especially together with palpitations, chest pain, breathlessness, fainting, diabetes or known heart disease. Seek urgent care for fainting, chest pain or a very fast irregular pulse.
References
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation. 1996;93(5):1043–1065. doi:10.1161/01.CIR.93.5.1043
- Kleiger RE, Miller JP, Bigger JT Jr, Moss AJ. Decreased heart rate variability and its association with increased mortality after acute myocardial infarction. Am J Cardiol. 1987;59(4):256–262. doi:10.1016/0002-9149(87)90795-8
- Nolan J, Batin PD, Andrews R, et al. Prospective study of heart rate variability and mortality in chronic heart failure: results of the United Kingdom Heart Failure Evaluation and Assessment of Risk Trial (UK-HEART). Circulation. 1998;98(15):1510–1516. doi:10.1161/01.CIR.98.15.1510
- Umetani K, Singer DH, McCraty R, Atkinson M. Twenty-four hour time domain heart rate variability and heart rate: relations to age and gender over nine decades. J Am Coll Cardiol. 1998;31(3):593–601. doi:10.1016/S0735-1097(97)00554-8
- Nunan D, Sandercock GRH, Brodie DA. A quantitative systematic review of normal values for short-term heart rate variability in healthy adults. Pacing Clin Electrophysiol. 2010;33(11):1407–1417. doi:10.1111/j.1540-8159.2010.02841.x
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health. 2017;5:258. doi:10.3389/fpubh.2017.00258
This article is for general information and does not replace a consultation with a doctor. HRV values depend strongly on the device, recording length and conditions; interpret them together with a healthcare professional.
Published on 4 June 2019