Low heart rate variability (HRV) does not cause symptoms by itself: HRV is a measurement. But a low reading often comes with things you do feel, because the short night, cold, hard training, stress or drinks that pull HRV down also make you tired, achy or tense. Some symptoms need a doctor whatever your HRV shows.
Why you cannot feel your HRV
HRV is how much the time from one heartbeat to the next changes, often by only a few hundredths of a second. Your watch, ring or strap works it out from the timing of your beats. You do not feel those changes.
What you can feel is your heart racing, pounding, fluttering or skipping a beat. Those are palpitations, a symptom, and they are separate from HRV. An extra beat that the software does not filter out can even push a reading up rather than down. Artifacts and extra beats explains how.
What you may feel on a low-HRV day
A low reading usually travels with a cause you can name, and the cause brings the symptoms:
| What lowered your HRV | What you may notice |
|---|---|
| A short or broken night | Tiredness, poor focus |
| A cold or other infection starting | A sore throat, aches, feeling run down |
| Hard training | Heavy legs, sore muscles, sessions that feel harder |
| Stress or worry | Tension, a racing mind, poor sleep |
| Alcohol the evening before | A hangover, poor sleep |
In one study, 15 adults who slept only 3 hours a night for three nights had lower RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD (a common HRV measure) in their morning readings. A review of 37 studies found that HRV changed with psychological stress in most of them. In data from an app used by 28,175 people, who took a short phone reading each morning after waking, HRV dropped by about 3% after moderate or hard training, about 10% when sick and about 12% after heavy drinking, each person compared with their own average.
Hard training does not always pull HRV down. In a study of 21 trained male triathletes, the 13 who did three weeks of overload training all did worse in a running test, and their weekly average HRV rose more than in the 8 who trained normally. Can HRV detect overtraining? covers this.
The number cannot tell you the cause
The authors of the app study called HRV sensitive but not specific. It reacts to many kinds of strain, so a low number alone cannot say which one is behind it. How you feel is often the better clue: a sore throat points to a cold, heavy legs to training. Can HRV warn you that you’re getting sick? and why is my HRV good but I feel tired? look at both sides of this.
What to do with a low reading
- Think back over the last day or two. Sleep, drinks, training, stress and a coming cold are common causes of a dip.
- Compare with your own normal. A 7-day average against your usual range says more than one morning. What does a low HRV mean? shows how to read it.
- Let your symptoms decide. If you feel unwell, rest, whatever the reading. If you feel fine, an easier day and an earlier night are enough.
- Watch the trend. If your HRV stays well below your usual range for weeks with no clear reason and you feel unwell or unusually tired, talk to a doctor.
When to see a doctor, whatever your HRV
HRV cannot diagnose anything, and a normal reading does not rule anything out. Go by your symptoms. Following advice from MedlinePlus and the NHS, get emergency help for:
- chest pain or discomfort that does not go away, or that spreads to your arm, neck, jaw or back
- fainting (passing out)
- severe trouble breathing, or a chest that feels tight or heavy
- palpitations that do not go away, or that come with chest pain, shortness of breath, dizziness, feeling faint or fainting
- unusual sweating with any of these
Contact a doctor promptly if:
- palpitations are new, keep coming back or last longer than a few minutes, or you often feel extra beats
- your resting pulse Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate is over 100 and exercise, anxiety or fever does not explain it
- you are more short of breath than usual
- you have felt tired for a few weeks and are not sure why, or tiredness is affecting your daily life
Bring your HRV trend if you like, but let the doctor decide what tests you need. The answer to what does a low HRV mean? ends with the same advice.