HRV training: how to use HRV for recovery, stress, and

HRV Training: a complete guide to heart rate variability to improve recovery and performance

SEO title: HRV Training: how to use HRV for recovery, stress, and performance

SEO description: A complete guide to HRV: what it is, what it measures, how to interpret it, and how to use it to guide training and recovery. Reliable measurement, decision tables, limitations, and FAQs.

Excerpt: HRV is not a fitness score: it is a window into the dialogue between stress and recovery. In this guide: physiology, measurement, interpretation, and practical application in training.


Why we are talking about HRV today: the paradox of data and the missing physiology

In recent years, HRV (Heart Rate Variability) has become a daily number: you look at it in the morning, compare it with yesterday, and use it to decide whether to “push” or “take it easy.” This is the paradox: the more data we have, the stronger the temptation to reduce a complex system to a simple traffic light.

Heart rate variability, however, was not born as a performance score. It is a signal of regulation. It tells the story of how the autonomic nervous system, cardiovascular reflexes, and breathing modulate the interval between one heartbeat and the next, beat by beat. It is a window into the dialogue between stress (training, work, sleep, inflammation, energy constraints) and recovery (rest, rebuilding, autonomic stability).

The second paradox is that the same value can mean different things in different people. Two athletes can have very different HRV values and both be doing well. The decisive variable is not “how” high or low it is in absolute terms, but how it moves relative to a personal baseline and in what context it appears (sleep, resting heart rate, symptoms, recent load).

Finally, there is a subtler cultural mistake: confusing measurement with control. HRV does not lend itself to linear optimization, because it responds to dynamics that include biological autonomy, circadian rhythms, allostatic load, immune status, and emotional variability. Its greatest practical value is helping you make more reasonable decisions over time: training hard when the system supports it, easing off when it signals fragility, and above all avoiding the accumulation of weeks of unprocessed stress.

The goal of this guide: to build a sustainable decision-making model for using HRV in training, recovery, and the management of physiological stress, without turning a useful signal into a new form of control anxiety.

What HRV (Heart Rate Variability) is

HRV is the variability of the intervals between consecutive heartbeats. If you measure the time between one beat and the next (RR or NN interval), you discover that it is not constant: even at rest, the heart is not a metronome.

This variability is physiological. It is a sign that heart rhythm is being modulated in real time by multiple systems: the autonomic nervous system (sympathetic and parasympathetic), the baroreflex (response to changes in blood pressure), breathing, metabolic state, and central signals related to stress and alertness.


Why the heart does not beat in a perfectly regular way

The sinoatrial node generates the electrical impulse that starts each heartbeat, but its rate is continuously modulated. Part of this modulation is rapid (seconds) and is often linked to the parasympathetic system and breathing; part is slower (minutes to hours) and integrates stress state, temperature, hydration, available energy, and other constraints.

In other words: variability is not noise. It is information about regulation.

The difference between heart rate and heart rate variability

Heart rate (HR) and HRV describe two different dimensions of the same system:

• HR: how many beats per minute. It is a “level” indicator (how fast the system is running at that moment).
• HRV: how much the interval between beats varies. It is a “modulation” indicator (how finely the system can adapt, shift gears, oscillate).

It is possible to have a similar HR with different HRV. For example, two mornings at 55 bpm may correspond to a day of successful recovery or to a day of poorly compensated stress. HRV does not “diagnose” this on its own, but it adds an important dimension: the flexibility of autonomic regulation.

A key concept: HRV as “room to maneuver”

In simplified terms, a higher resting HRV tends to be associated with a greater capacity of the cardio-autonomic system to adapt to stimuli. It does not mean “you are stronger today”; it more often means: “the system has regulatory margin.” But this margin must always be interpreted in relation to trends, context, and bodily signals.

The autonomic nervous system and HRV

The most useful reading of HRV goes through the autonomic nervous system (ANS), which regulates automatic functions: heart rate, blood pressure, intestinal motility, thermoregulation, vascular tone. It is not a simple switch, but a system of continuous balancing between the demands of mobilization and the need for recovery.

Sympathetic: mobilization and physiological cost

Sympathetic activation increases energy availability, alertness, and responsiveness. In many conditions, an increase in sympathetic drive is associated with:

• increased heart rate
• reduced beat-to-beat variability
• greater rigidity of the autonomic signal (fewer rapid oscillations)

This is not “bad” in itself: it is adaptive during exertion, acute stress, and competition. It becomes problematic when it is chronic or when it is added to insufficient recovery.

Parasympathetic (vagal): recovery and rapid modulation

The parasympathetic system, especially through the vagus nerve, rapidly modulates the heart (within seconds). At rest, a stronger vagal tone tends to be associated with:

• higher HRV (especially in indices sensitive to rapid components)
• lower heart rate
• better transitions between states (from stress to recovery)

But interpretive maturity matters here as well: “more vagal” is not always synonymous with “better” if the context suggests fatigue, infection, or downregulation after heavy weeks.

Balance and co-activation: it is not on/off

Sympathetic and parasympathetic activity can coexist and modulate different organs in different ways. The heart is one of the most “readable” districts, but it is not the whole organism. This is why HRV should not be interpreted as the total sum of health or readiness: it is a partial signal, but often a very sensitive one.

Baroreflex and blood pressure

The baroreflex regulates heart rate in response to changes in blood pressure. If pressure rises, the system tends to slow the heart; if it falls, it tends to speed it up. This contributes to oscillations in RR intervals and links HRV to cardiovascular dynamics, not just to “mental stress.”

Breathing and RSA (respiratory sinus arrhythmia)

During inhalation, heart rate tends to increase slightly; during exhalation, it tends to decrease. This phenomenon (RSA) is an important component of resting HRV. It is one of the reasons why breathing during measurement is a confounder: if you control it or deliberately slow it down, you can modify the signal and make the data less comparable.

What HRV really measures: main indices and physiological meaning

Saying “HRV” is like saying “temperature”: you need to specify how you are measuring it. There are different indices, some more suitable for daily practice, others more useful in research or clinical evaluation settings.

Time domain: RMSSD and SDNN

• RMSSD: reflects mainly short-term variability and is often used as a proxy for parasympathetic modulation at rest. It is a very common metric for daily monitoring.
• SDNN: measures the overall variability of NN intervals within a time window. In short recordings it is less specific; in long recordings (hours) it integrates multiple components and can take on different meanings.

Frequency domain: HF, LF, VLF (and why LF/HF is often fragile)

In spectral analysis, variability is broken down into frequency bands:

• HF: often associated with RSA and parasympathetic influence.
• LF: historically interpreted as “mixed” (sympathetic + parasympathetic + baroreflex). Reducing it to “sympathetic” is often an imprecise oversimplification.
• VLF: very slow components, influenced by thermoregulation, hormones, inflammation, and other factors; in short measurements it is difficult to interpret well.

The LF/HF ratio has been used as an index of sympathetic-parasympathetic balance, but in many real-world conditions it is unstable and sensitive to confounders (breathing, posture, measurement duration). In daily training practice, it is often more robust to focus on a repeatable metric (such as RMSSD or its logarithmic version) and on longitudinal trends.

Non-linear measures (briefly)

There are metrics that attempt to describe the complexity of the signal (for example entropy and other measures). They can be interesting for describing adaptability, but they rarely add decision-making value for the individual athlete compared with consistent monitoring of simple, well-standardized metrics.

Which metric to use in practice

For daily use aimed at training and recovery, the priorities are: consistency of measurement, repeatability, trend-based interpretation. In many cases RMSSD (or logRMSSD) is a practical choice because it is sensitive to rapid components and relatively robust in short measurements, if well standardized.

Why HRV matters for health

In a non-reductionist framework, HRV is often considered a marker of physiological resilience: the system’s ability to maintain stability while changing, that is, to adapt without becoming rigid.

HRV, chronic stress, and allostatic load

Stress is not just an event; it is also a regulatory dynamic. When the body has to maintain performance and vigilance for weeks without adequate recovery, allostatic load increases: the physiological cost of adaptation. In many people this is reflected in lower HRV compared with their normal pattern, together with fragmented sleep, increased resting heart rate, worse recovery, irritability, or cognitive fatigue.

Connections with sleep, inflammation, and metabolism

Insufficient sleep, infections, inflammation, and energy restriction tend to modify autonomic tone and therefore the HRV signal. This does not mean HRV directly measures inflammation or glycogen; it means autonomic regulation is one of the “levels” on which these states leave their mark.

High HRV is not synonymous with “health”

Higher HRV may be associated with good autonomic balance at rest, but health is a broader clinical picture: body composition, blood pressure, metabolism, mental state, medical history, medications, life rhythms. HRV is most useful as a longitudinal monitor to recognize derailments: when your system changes persistently compared with how it usually behaves.

HRV and training: what it reflects and what it cannot tell you

For people who train, the implicit promise of HRV is: “it tells you if you are ready.” Reality is more mature: HRV says something about systemic recovery and autonomic regulation. This is important, but it does not always coincide with local muscular recovery or the technical quality of a session.

Muscular recovery vs systemic recovery

After a strength session with a lot of eccentric work, muscle damage and DOMS may be high even if the autonomic system is already settling back. Conversely, after a period of work stress and poor sleep, the muscles may “feel okay” but the autonomic system shows fragility. HRV more often speaks to the second scenario.

Internal load vs external load

External load is what you do (kilometers, watts, sets, kg). Internal load is how the body experiences it: heart response, neuroendocrine stress, perceived exertion, sleep disruption. HRV is a bridge because it can reflect the disturbance of internal load even when the external load seems “normal.”

Effects of intensity and volume

• High-intensity sessions (HIIT, races, interval training) often reduce HRV in the following 24–48 hours.
• High volume (long endurance, accumulated load) can reduce HRV more gradually and persistently, especially if energy availability is low.
• Well-managed blocks may show: acute drop, stabilization, then “rebound” during deload (autonomic supercompensation), but this is not guaranteed and depends on context.

Typical differences between disciplines

In endurance sports, HRV can be particularly informative for monitoring the systemic cost of volume and sleep. In strength training, HRV may remain relatively stable even with significant neuromuscular fatigue; it becomes more sensitive when the total load (training + life) exceeds recovery capacity. In intermittent sports (team sports, combat sports), the alternation of neuromotor, cognitive, and metabolic stress makes HRV useful, but only if it is placed within a monitoring system that also includes symptoms and performance.

Practical examples (pattern-based reading)

• Loading week: HRV tends to drop or become more unstable; if sleep and RHR are good, this may be a “functional” drop.
• Deload week: HRV often rises back toward baseline or above; more than the peak, regularity matters.
• Return after a break: HRV may be high but muscular and tendon tolerance is low: risk of overestimating mechanical readiness.

High HRV and low HRV: what they really mean

“High HRV = good, low HRV = bad” is a childish reading of an adult signal. The meaning is always relative to three elements: personal baseline, trend, and context.

Low HRV: common physiological interpretations

A value lower than your norm may reflect, among other things:

• greater sympathetic activation (acute stress, anxiety, pain)
• incomplete recovery (training + insufficient sleep)
• metabolic stress (energy restriction, dehydration, unstable blood glucose)
• inflammation or an incoming infection

But it can also simply be individual variability or a noisy measurement (different posture, controlled breathing, artifacts).

High HRV: useful, but not always a “green light”

A value higher than usual may indicate good recovery and greater vagal modulation. However, in some phases of prolonged fatigue, unusually high HRV may appear together with declining performance and a feeling of “flatness”: a possible form of rebound or parasympathetic predominance in a system that is reducing arousal. It is not a sign to fear automatically; it is a sign to correlate with performance, energy, and sleep.

Consistency with resting HR and perception

HRV and resting heart rate (RHR) often move in opposite directions: HRV down and RHR up may suggest systemic stress. But it is not a law. This is why it is useful to integrate subjective signals as well: energy, sleep quality, desire to train, “unusual” aches, irritability.

How to measure HRV: when, how, and with what data quality

Data quality comes before interpretation. If you measure under different conditions every day, HRV becomes more an indicator of methodological chaos than of physiology.

When to measure it: why morning is the cleanest window

The most interpretable measurement is in the morning right after waking, before caffeine, work, conversations, bright artificial light, or training. In that window, autonomic state is less influenced by acute stimuli, and the data become more comparable.

Standardization: posture, duration, environment

• Posture: choose supine or seated and keep it the same over time.
• Duration: use a consistent duration (often a few minutes are enough if the signal is clean).
• Breathing: natural, without forcing slow rhythms or techniques.
• Environment: similar temperature, no talking, no moving.

Signal quality: artifacts, ectopic beats, movement

Artifacts and ectopic beats can alter RMSSD and other metrics. Signal cleanliness is crucial: if you move, cough, tighten the strap, or the sensor loses contact, HRV can change without your physiology having changed.

Trend vs single measurement

A single value is fragile. HRV should be read over a 7–14 day window to distinguish normal daily variation from a persistent change. A useful initial baseline requires at least 1–2 weeks of consistent measurements; a robust baseline is built over weeks, including different periods (loading, deloading, work stress).

Checklist: reliable measurement (quick)

• Measure in the morning, right after waking, before caffeine and screens.
• Same posture every day (supine or seated).
• Same measurement duration.
• Breathe naturally, without intentional “resonance breathing.”
• No movement, no talking.
• If the measurement is disturbed (coughing, interruptions), repeat it or make a note.
• Always evaluate trend and context, not the single number.

How to use HRV to guide training (without becoming a slave to the number)

The most mature use of HRV is as a tool for autoregulation within a sensible plan. It does not replace programming, progression, and periodization; it complements them, especially on days when real life interferes (poor sleep, work stress, travel, illness).

“Traffic light” approach based on deviation from baseline

A simple but useful model is to think in terms of deviations from your baseline:

• Green: HRV near or above baseline, stable RHR, good feelings.
• Yellow: HRV moderately below baseline or more unstable; mixed signals.
• Red: HRV clearly below baseline for several days and/or elevated RHR, terrible sleep, obvious symptoms.

It is not a dogma: it is a minimal grammar for reducing impulsive decisions.

Intense training

In general, intensity is more sustainable when the autonomic system is stable: HRV within normal range, RHR not elevated, decent sleep, motivation present. If HRV is low but everything else is okay and the drop is expected (for example the day after a hard session), it may make sense to keep the session but reduce volume or density.

Moderate training

On “yellow” days, the smartest choice is often to keep a moderate stimulus: technique work, easy-moderate aerobic work, submaximal strength with some reserve, work that feeds continuity without adding too much systemic debt.

Active recovery and mobility

Active recovery is not a ritual: it is a choice of light stimulus that often supports circulation, mood, and autonomic stability without adding stress. Walking, easy cycling, mobility, natural breathing, and daylight are simple tools when the system is “noisy.”


Rest days: when rest is an intervention

Rest becomes strategic when HRV is persistently low and/or RHR is high, sleep is disturbed, and performance is declining. Here, “not training” is not giving up: it is a physiological input that allows stress to be reabsorbed and regulation to be restored.

If you want to understand in practical terms how to apply HRV to training management, you can also read our in-depth article on how to use HRV for training.

HRV and overtraining: from acute load to systemic fatigue

The term “overtraining” is often used to refer to any tiredness. In reality, there are different levels:

• Functional overreaching: temporary programmed fatigue, followed by recovery and improvement.
• Non-functional overreaching: prolonged performance decline, slow recovery, high systemic stress.
• Overtraining syndrome: a complex and multifactorial condition, rare but serious, with persistent alterations in performance and well-being.

Typical signs that may appear in the data

• Persistently low HRV relative to baseline, with higher RHR.
• Unusually high HRV but declining performance and a feeling of being “switched off” (not frequent, but possible).
• disturbed sleep, reduced exercise tolerance, worse mood, loss of appetite, or marked cravings.

Plausible mechanisms (without shortcuts)

When load exceeds recovery capacity, multiple levels come into play:

• neuroendocrine stress and altered rhythms (cortisol and arousal)
• inflammation and oxidative stress linked to volume/intensity and insufficient recovery
• energy depletion (especially with low carbohydrate availability or chronic calorie deficit)
• cognitive load and psychosocial stress that amplify internal cost

Why HRV alone does not diagnose

HRV is not a diagnostic tool for overtraining. It is an indicator of regulation. Diagnosis (or even just the decision to seriously deload) requires integration: performance trends, symptoms, appetite, sleep, pain, and in some people variables such as menstrual cycle or endocrine conditions. HRV is useful as an alarm bell, not as a judge.

Monitoring framework (simple but complete)

• HRV/RHR trends: not the single day, but the direction over 1–3 weeks.
• Performance: same tests or same indicators (paces, watts, loads at similar RPE).
• Symptoms: sleep, mood, unusual aches, motivation.
• Real recovery: hours of sleep, rest days, sufficient nutrition.

Factors that influence HRV: interpreting the signal in the real world

HRV is sensitive. This is both an advantage and a limitation: it responds to many factors, some important (illness), others trivial (inconsistent measurement). The skill lies in understanding what, in your case, moves the signal the most.

Sleep

Sleep duration, continuity, and timing influence autonomic tone. A fragmented night can reduce HRV and increase RHR even if total hours seem “acceptable.” In addition, going to bed very late (circadian misalignment) can destabilize the signal in the following days.

Psychological stress and cognitive load

The body does not perfectly distinguish between physical stress and mental stress: both require regulation, energy, and vigilance. Periods of high-pressure work, conflict, or rumination can reduce HRV even if training is stable.

Nutrition and energy availability

Chronic calorie deficit, disordered meal timing, or insufficient post-workout recovery may be reflected in lower HRV and higher RHR. The variable is not “eating perfectly,” but ensuring that training load is supported by adequate energy and nutrients.

Alcohol

Alcohol tends to worsen sleep architecture and autonomic regulation: higher RHR and suppressed HRV are often observed during the night and the following morning, even when the subjective feeling is “I slept.”

Training

Different stimuli have different signatures:

• HIIT: sharper drop in the following 24–48 hours.
• High volume: progressive drop, especially if energy recovery is insufficient.
• Heavy strength work: variable; depending on the person, it may affect sleep/pain more than HRV.
• Competitions: often marked alterations with individual recovery times.

Illness and inflammation

Many people notice an HRV drop (and RHR increase) before symptoms appear. It can be an “early warning,” but it is not specific: stress and poor sleep can also produce similar patterns. It is a signal that invites caution, not conclusions.

Travel and jet lag

Circadian misalignment alters sleep, temperature, appetite, and autonomic regulation. During travel days, HRV can become unstable: in those periods it is more useful to lower interpretive ambition and return to standardization once the internal clock realigns.

How to improve HRV over time: strategy, not obsessive optimization

“Improving HRV” should not become a project in itself. In most cases, when you improve life rhythm, sleep, training progression, and energy recovery, HRV tends to stabilize or improve as a side effect. The useful direction is to build a system that reduces the need to chase the number.

Sleep as the primary lever

Sleep is often the most decisive and most underestimated variable. Not just “hours,” but regularity and quality:

• fairly consistent schedules
• natural light in the morning to reinforce circadian entrainment
• reduction of intense evening stimuli (cognitive work, strong light, conflict)
• a cool room and a simple routine


Stress management: simple tools, not anxious rituals

Slow breathing can increase HRV during the breathing exercise, but the goal is not to “raise the number”: it is to move the system toward a recovery state when you are in hyperarousal. A break, a short walk, light exposure, cognitive decompression, and work boundaries are often worth more than techniques performed compulsively.

If stress is persistent and structural (timing, responsibilities, decision load), HRV can become an indicator that what needs to change is structure, not the addition of more tools.

Training: progression and intensity distribution

Resilience is built through well-dosed load and sufficient recovery:

• gradual progression (volume/intensity)
• periodic deload weeks
• intelligent intensity distribution (do not turn every session into a test)
• strength training as support for robustness (tendons, posture, movement economy)

Recovery: energy, hydration, light movement

Recovery does not just mean “staying still.” It means creating the conditions for repair and stability:

• protein and calorie intake adequate to the load
• sufficient carbohydrates when volume requires them
• hydration and mineral salts (especially if you sweat a lot)
• walking and daylight
• reduced alcohol when recovery is fragile


Checklist: 10 high-impact, low-complexity actions

• Measure HRV the same way every time (to make the data useful).
• Protect 30–60 minutes of evening “landing time” without heavy cognitive work.
• Expose yourself to natural light within 1 hour of waking.
• Avoid stacking 3–4 consecutive days of high intensity.
• Insert planned deloads (not only “when you are destroyed”).
• Train easy aerobic work: it is often an investment in autonomic stability.
• Reduce alcohol during loading periods or when HRV/RHR signal fragility.
• Post-session recovery: adequate meals, hydration, prioritized sleep.
• Note 2–3 context variables (sleep, perceived stress, DOMS) to give meaning to HRV.
• Decide based on trends and consistency, not on fear of a single data point.

Limits of HRV: what it cannot replace

HRV is a useful measure, but it is not a total measure. Its main limitation is overinterpretation: asking an autonomic signal to answer questions that concern mechanics, technique, and complete psychology.

What it does not measure directly

• local muscle damage and specific microtrauma
• glycogen and precise energy status (it may reflect them indirectly)
• neuromuscular readiness reliably for everyone
• technique and movement quality
• motivation and complete psychological readiness

Sensitivity to confounders

Breathing, posture, measurement duration, sensor noise, ectopic beats, caffeine, and acute stress can alter HRV. If you do not standardize, the metric becomes more an indicator of “measurement conditions” than of physiology.

Risk of reactive interpretation

The practical risk is changing everything because of a single reading. This creates more instability in training than it prevents problems. HRV works when it supports continuity: small adjustments, not daily revolutions.

Interindividual differences

Age, sex, training level, genetics, medications, menstrual cycle, and medical conditions modify the ranges. Comparing yourself with “ideal values” online is almost always useless. The useful comparison is with yourself, over time.

When HRV can be misleading (and how to protect yourself from mistakes)

HRV is interpretation, not direct reading. Some situations produce ambiguous signals. Knowing in advance where mistakes happen reduces impulsive decisions.

High HRV but high fatigue

If HRV is high but you feel drained and performance is dropping, do not use the data as automatic permission. You may be in a rebound phase after stress or in a form of fatigue in which arousal is “low” but the capacity to produce work has not returned. In these cases, it is wiser to test with a moderate session and evaluate the response.

Transient low HRV

A drop the day after HIIT or after a short night is common. It is not necessarily a sign of “never train.” It may be an invitation to avoid a second intense hit at close range and to choose a stimulus with a lower systemic cost.

Controlled breathing during measurement

If you intentionally slow your breathing, you can artificially increase RMSSD and make days non-comparable. If you choose guided breathing, it must always be identical; otherwise it is better to breathe naturally and measure what is really happening.

Non-standard measurements (after a shower, caffeine, after getting up)

Morning is not “morning” if you have already started the day. Simply getting up, talking, checking your phone, or drinking coffee can shift autonomic tone and change the value. If the routine varies, HRV becomes variable.

Highly trained athletes vs beginners

Highly trained athletes may have higher HRV and greater physiological oscillation; beginners may have lower HRV without that meaning “you are in bad shape.” In addition, in the first months of consistent training, HRV may be unstable while the body learns to tolerate load and recovery.

Anti-error framework: 3 questions before changing the plan

• Trend? Is this a persistent change or a single day?
• Context? Sleep, alcohol, stress, travel, cycle, illness: what has changed?
• Confirmations? RHR, feelings, recent performance: are they consistent with HRV?

If two out of three point in the same direction, the decision is more robust. If not, it is often better to keep the structure and make only small adjustments.

FAQ: frequently asked questions about HRV and training

What is HRV and what does it really measure?

HRV (Heart Rate Variability) measures the variability of the time intervals between one heartbeat and the next (RR/NN intervals). It does not describe how “strongly” the heart beats, but how finely the autonomic nervous system modulates heart rhythm in response to breathing, blood pressure, stress, and recovery.

Low HRV: what does it mean?

In general, HRV that is lower than your own baseline signals reduced autonomic modulation, often associated with physiological or psychological stress, insufficient sleep, high training load, impending illness, or incomplete recovery. It should be interpreted together with trends, resting heart rate, and context.

Is high HRV always positive?

Not always. HRV that is higher than usual may indicate good recovery and high vagal tone, but in some situations it may appear as rebound after stress or during phases of fatigue in which performance worsens even though the value seems “good.” Consistency with energy, sleep, and performance is decisive.

When should HRV be measured to get reliable data?

Ideally in the morning right after waking, before caffeine, training, and intense stimuli, under standardized conditions (same posture, same duration, natural breathing). The goal is to reduce noise and make day-to-day measurements comparable.

How should I use HRV in training in a practical way?

Use it to modulate load relative to your baseline: if the value is stable and subjective signals are good, intensity is more sustainable; if it is falling and resting heart rate is higher than normal, it may make more sense to reduce intensity or volume and prioritize recovery. The point is not to react to a single data point, but to read trends and consistency with symptoms and performance.

How many days are needed to build a useful baseline?

In practice, a window of at least 1–2 weeks of consistent measurements is needed to have an initial reference, but a baseline becomes more robust over 3–6 weeks, especially if you include different periods (loading, deloading, work stress, sleep variation).

Should I control my breathing during measurement?

It is preferable to breathe naturally and consistently. Slow breathing and