Heart Rate Variability (HRV)

About the author

Dr. Yvo Sijpkens is an internist at Haaglanden Medical Center in The Hague and medical advisor to Stichting Je Leefstijl Als Medicijn. He specializes in lifestyle medicine and its application to conditions such as diabetes and metabolic dysfunction.

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In brief

• HRV is the variation in time between heartbeats and reflects the balance between gas (sympathetic) and brake (parasympathetic)
• With metabolic dysfunction, HRV is on average lower; successful TKB can be accompanied by gradual increase
• The trend within the same person is more important than the absolute value or comparison with others
• A daily morning measurement combined with notes about sleep, nutrition, and stress makes HRV useful as feedback
• HRV is not a goal in itself, but an extra window on the effects of lifestyle interventions

1. What is HRV?

Heart rate variability (HRV) is the variation in time between consecutive normal heartbeats, measured in milliseconds. At a heart rate of 60 per minute, there is on average one heartbeat per second, but in reality the interval is sometimes 0.92 seconds and then 1.08 seconds. That subtle variation is HRV.

HRV mainly reflects the balance between two parts of the autonomic nervous system:

• Sympathetic (gas): activation, stress response, alertness
• Parasympathetic (brake): recovery, rest, digestion, sleep

A flexible nervous system shows a healthy degree of variation: HRV fluctuates with load, recovery, emotions, sleep, and illness. A clearly reduced HRV over longer periods fits with an overloaded stress system or autonomic dysfunction.

2. HRV and metabolic dysfunction

With metabolic dysfunction (insulin resistance, metabolic syndrome, prediabetes, type 2 diabetes, non-alcoholic fatty liver, belly fat), we often see:

• Reduced parasympathetic activity
• Increased sympathetic activation
• Disrupted day-night rhythms (sleep-wake, cortisol, eating pattern)
• Chronic low-grade inflammation

All of this often goes together with lower HRV. Research shows that people with metabolic syndrome and type 2 diabetes on average have lower HRV than metabolically healthy controls. This fits with autonomic dysfunction and higher cardiovascular risk.

HRV as supplementary signal

HRV is not a diagnosis but an extra signal that says something about the stress system load, quality of recovery, and degree of autonomic dysfunction. HRV does not replace classic risk assessment with blood pressure, glucose, lipids, smoking status, and family history, but can be a supplementary feedback tool within lifestyle treatment.

3. HRV in the context of TKB

Therapeutic carbohydrate restriction (TKB) and ketogenic metabolic therapy focus on:

• Normalization of glucose level and insulin
• Reduction of visceral fat
• Lowering of inflammation
• Stabilization of energy and hunger regulation

If these interventions are effective, one may indirectly also expect improvement of the autonomic nervous system:

• Less chronic sympathetic overactivation
• More room for parasympathetic recovery
• Often better sleep quality and less nocturnal hypo- and hyperglycemia
• Decrease of inflammatory stimuli

In practice, this can manifest as a gradual rise in average HRV over weeks to months, a lower resting heart rate, and more stable day-to-day fluctuations.

Important

HRV here is a result, not a primary goal. The clinical goal remains improvement of metabolic markers, complaints, quality of life, and risk of complications.

4. How to measure HRV in practice

For most people, a simple, standardized home measurement suffices.

Recommendations for a practical morning measurement

• Preferably use a chest strap with HRV-suitable app or a proven reliable wearable
• Measure once per day, preferably in the morning after waking
• Measure in lying or sitting rest, for 3 to 5 minutes
• Use as much as possible the same conditions: before coffee, cigarette, and meal, and not immediately after fever or acute illness

Comparison

The absolute value of HRV is not well comparable between different devices and algorithms. Therefore, compare yourself mainly with yourself over time and not with others’ numbers.

RMSSD and Body Battery

RMSSD is a commonly used HRV measure. The abbreviation stands for Root Mean Square of Successive Differences. The meter looks at the differences in time between consecutive heartbeats, squares them, takes the average, and then the square root. RMSSD mainly reflects parasympathetic (vagal) activity and is relatively stable for a short morning measurement. In practice, the trend of RMSSD over time is more important than the absolute value.

Body Battery (like Garmin) is a composite score of 0 to 100. This score is not a pure HRV measure but an algorithm that combines HRV, heart rate, sleep, and activity into an estimated energy balance. A higher Body Battery fits with more recovery and reserve, a low score with accumulated fatigue. Here too, mainly the trend within the same person is relevant.

When HRV is not suitable

HRV is not suitable as a leading parameter with:

• Known heart rhythm disorders such as atrial fibrillation
• Pacemaker dependency
• Clear irregular rhythms reported by the device without cardiological interpretation
• Acute cardiac complaints such as chest pain, severe shortness of breath, or collapse

In these situations, medical assessment always takes precedence over HRV interpretation.

5. How to interpret HRV

The main question is not whether HRV is good, but how HRV develops over time in relation to lifestyle and complaints.

Practical steps

Step 1: First build a baseline over 2 to 4 weeks with a daily morning measurement and short notes about sleep, stress, exercise, TKB compliance, alcohol, and illness.

Step 2: Then look at trends:

Trend	Characteristics
Favorable	Gradual rise or stable, relatively high HRV; falling or stable low resting heart rate; subjectively better recovery and more stable energy and glucose
Less favorable	Persistently low or falling HRV; rising resting heart rate; more complaints such as fatigue, poor sleep, and fluctuating glucose values

Day-to-day variation is normal. Several consecutive days of clearly lower HRV and higher resting heart rate, in combination with sleep deprivation, illness, high stress, or excessive training, indicate insufficient recovery.

6. Lifestyle advice for HRV and TKB

The goal of lifestyle interventions is not a magic HRV number, but improvement of metabolic health, complaints, and quality of life. HRV can serve as supplementary feedback.

1. Sleep and circadian rhythm

• Fixed bedtime and wake time, even on weekends
• Aim for 7 to 9 hours of sleep per night
• Cool, dark, and quiet bedroom, without screens in the last hour
• Avoid late heavy meals, especially with lots of carbohydrates, alcohol, and intensive exercise late in the evening

Within TKB, a last meal that is easily digestible, contains protein and fat, and doesn’t cause a large glucose spike helps limit nocturnal dysregulation and HRV decline.

2. Exercise, training, and VO2max

• Daily at least 30 minutes moderate intensity movement, such as walking or cycling
• 2 to 3 times per week strength training for major muscle groups
• Gradually build up intensive training and dose well

Note when starting TKB

When starting or scaling up TKB, it’s wise not to simultaneously maximize training intensity in the first weeks. With falling HRV, rising resting heart rate, and increasing fatigue, temporary reduction of training volume or intensity may be needed.

3. Nutrition and metabolic strategy

Within TKB and ketogenic therapy:

• Focus on whole, unprocessed food with sufficient protein, healthy fats, and micronutrient-rich sources
• Limit fast sugars, refined carbohydrates, ultra-processed food, and frequent alcohol
• Ensure adequate energy and protein intake to prevent malnutrition and muscle mass loss

Watch for patterns where HRV drops after alcohol, serious glucose dysregulation, or clear relapse in compliance with the nutrition plan. Use this as extra feedback alongside glucose values, weight, and complaints.

4. Breathing regulation and HRV biofeedback

Simple protocol, provided there are no cardiovascular contraindications:

• 1 to 2 times per day, and extra with stress, 5 to 10 minutes of calm breathing
• Sitting or lying, relaxed posture
• Approximately 4 to 5 seconds calm inhale through the nose
• Approximately 5 to 6 seconds exhale through nose or slightly pursed lips
• Total breathing frequency around 6 breaths per minute

This can promote parasympathetic activity, acutely increase HRV, and facilitate transition to recovery or sleep.

5. Stress management and daily structure

• Plan short recovery moments throughout the day, for example 3 to 5 minutes calm breathing or a short walk
• Limit constant stimuli from phone and email by choosing fixed times for responding
• Build clear transition rituals between work and evening, and between evening and sleep

A better balance between load and recovery will often manifest in more stable HRV with fewer prolonged low points.

7. HRV: what to do and not to do

HRV is a valuable tool, but not a goal in itself.

Do

• Use HRV as supplementary feedback alongside glucose profiles, weight, waist circumference, blood pressure, lipids, and complaints
• Focus mainly on trends over weeks to months
• Link HRV to concrete lifestyle factors: sleep, TKB compliance, training load, and stress management
• Use HRV as a conversation opener in guidance, not as a hard threshold

Don’t

• Use HRV as the only or main steering instrument
• Make big decisions based on a single deviant measurement
• Compare yourself to HRV numbers of others or to fixed ideal values from apps
• Try to interpret cardiac complaints or clear rhythm disorders yourself with HRV data instead of medical assessment

8. Summary

• HRV is the variation in time between heartbeats and reflects the balance between gas and brake in the autonomic nervous system
• With metabolic dysfunction, HRV is on average lower; successful TKB and lifestyle improvement can be accompanied by a gradual rise in HRV
• The trend within the same person is more important than the absolute value or comparison with others
• A simple daily morning measurement, combined with sleep, nutrition, training, and stress notes, makes HRV a useful feedback instrument
• Sleep, exercise, nutrition, breathing regulation, and stress management are the main levers to improve both metabolic health and HRV

Key message

HRV is not a goal in itself, but an extra window on the effects of TKB and other lifestyle interventions on stress system and recovery capacity.

Veelgestelde vragen

What is heart rate variability (HRV)?

HRV is the variation in time between consecutive normal heartbeats, measured in milliseconds. It reflects the balance between the sympathetic (gas: activation, stress) and parasympathetic (brake: recovery, rest) nervous system. A healthy degree of variation indicates a flexible nervous system.

What does HRV say about metabolic health?

With metabolic dysfunction (insulin resistance, type 2 diabetes, fatty liver), we often see lower HRV. This fits with reduced parasympathetic activity, increased sympathetic activation, and chronic low-grade inflammation. HRV is an extra signal about stress load and recovery quality.

How do I measure my HRV at home?

Measure once per day, in the morning after waking, in lying or sitting rest for 3-5 minutes. Use a chest strap with HRV app or reliable wearable. Measure before coffee, cigarette, and meal. Compare yourself to yourself over time, not to others' numbers.

What is RMSSD and Body Battery?

RMSSD is a commonly used HRV measure that mainly reflects parasympathetic (vagal) activity. Body Battery (like Garmin) is a composite score of 0-100 that combines HRV, heart rate, sleep, and activity into an estimated energy balance. For both, the trend over time is more important than the absolute value.

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