Why Is My HRV Low? Common Causes and What the Evidence Shows

Why is my HRV low?

Heart rate variability (HRV) is the beat-to-beat variation in the interval between heartbeats. A low reading most often has an everyday explanation, such as a short night, alcohol, an infection, stress or a hard workout. Usually it is the body's normal reaction to that load, not a diagnosis.

This page is a map of the common causes, in the order below. For each one it gives what the evidence shows, how long the change typically lasts where that is known, and the page that covers it in depth. The evidence labels mean: shown — studies found the change; mixed — studies disagree, or only part of the question has been answered; not shown — studies looked and did not find it, which is not proof that it never happens; no data — we found no study that measured it.

Is your HRV really low?

Check this before looking for a cause. Healthy people differ enormously in HRV [S1], so a number means little until you compare it with your own baseline: your usual range, built from frequent readings rather than one [S2]. Compare like with like: the same device and the same measure, under the same conditions, such as your night-time average or a reading taken on waking. The input signal, the length of the recording, breathing and the analysis method all change the value [S3]. Devices do not all report the same measure — Apple Health records HRV as SDNN, and Apple Watch Series 12 and Ultra 4 on watchOS 27 show two HRV variants — Recovery HRV and Overall HRV — and measure HRV as often as every five minutes, while other devices may use RMSSD or their own scores — so numbers from two devices are not interchangeable [S3].

Many experimental, personal and environmental factors affect a reading and how reliable it is [S3], so a single low morning can be ordinary variation with no cause worth finding. More on this: your HRV baseline, what a single HRV value can and can't tell you, how wearables measure HRV, why HRV changes from day to day and, for one common device, why your Apple Watch HRV may look low.

What can lower HRV?

Sleep. In a meta-analysis of controlled sleep-deprivation studies, mostly run in laboratories, RMSSD fell, and the authors call the size of the drop relatively modest; the fall in SDNN was not significant [S4]. So a watch that reports SDNN may show a short night less clearly than a study that used RMSSD. How much one ordinary late night lowers your HRV, and for how long, has not been measured. Evidence: shown, as a small drop in RMSSD after sleep loss. How long: not known for an ordinary short night. More: HRV and heart rate during sleep.

Alcohol. After drinking, night-time HRV tends to be lower and resting heart rate higher, and the more you drink, the larger the change; a large real-world dataset and a small laboratory trial point the same way [S5] [S6]. In that dataset, a night with one drink more than a person's usual, compared with one fewer, came with HRV lower by 3.8 ms in women and 3.3 ms in men and night-time resting heart rate higher by 2.8 beats per minute in women and 2.4 in men [S5]. That study was funded by WHOOP, and several of its authors work for the company. There is no systematic review of alcohol and HRV yet. Evidence: shown. How long: the night after drinking was measured; how many nights it takes to return to your usual range was not. More: alcohol and HRV.

Illness. An infection or inflammation tends to lower HRV and raise resting heart rate. Across wearable studies, SDNN was lower when the inflammation marker CRP was raised in 83% of comparisons, and the reviewers call wearable HRV an exploratory biomarker, not a diagnostic tool [S7]. The best-studied early warnings ran on heart rate and steps rather than on HRV, and in a prospective smartwatch study, stress, alcohol and travel set off the same alerts as infection, only less often [S8]. That study's senior author co-founded and advises several health-technology companies. So a dip can be an early, non-specific sign that the body is under strain, but it does not tell you the cause. Evidence: shown, as a non-specific signal. How long: the days around the start of symptoms; in that study, signals came at a median of 3 days before symptom onset [S8]. After a vaccination, short HRV changes recovered within up to 3 days [S9]. More: illness and HRV.

Stress, short-term and long-term. A short, clear challenge lowers HRV: in one study of 67 ninth-grade students, a real mathematics exam lowered HRV and mood compared with a normal lesson [S10]. HRV comes back after such a task, but studies measure that recovery over windows of different length, so there is no single recovery time [S11]. Long-term load is less clear. In small studies of doctors, each using a different HRV device, HRV was lower during high-stress work than during recovery time, and the reviewers rate the studies' quality as moderate at best [S12]. In a large long-term study in Belgium, work stress was not linked with resting HRV, either at the end of follow-up or over the years [S13]. A single low HRV reading does not by itself mean you are stressed or unwell. Evidence: shown for short, acute stress; mixed for long-term stress. How long: short after an acute stressor; lower while a demanding period lasts; a lasting drop in resting HRV has not been shown. More: acute stress and HRV and cortisol and the stress response.

Training. A hard workout lowers HRV for a while, and it comes back on its own: after a single aerobic session, complete recovery of the heart's autonomic control takes up to a day after low-intensity exercise, one to two days after threshold-intensity exercise and at least two days after high-intensity exercise [S14]. Recovery is faster in fitter people, and strength training has been studied too little to give the same timeline [S14]. Over weeks, training that improved performance went with a small rise in resting RMSSD [S15]. Resting HRV, however, was largely unaffected by overreaching, that is, hard training with falling performance, so HRV on its own does not tell you whether you have overdone it [S15]. Evidence: shown for aerobic exercise; strength training is poorly covered. How long: hours to days, longer after harder sessions. More: exercise and HRV.

Menstrual cycle. On average, HRV is a little higher at the start of the menstrual cycle and a little lower toward the end, with differences in time-domain HRV of 3 to 9% across wearable studies; users of hormonal contraception had lower HRV, especially late in the cycle [S16]. In one cohort study with a fitness tracker, HRV fell before a period and rose again after it, in women with and without premenstrual disorders [S17]. The average is not everyone's curve: in one small study that lined cycles up to ovulation, each woman had her own pattern [S18], so compare the same cycle days across your own cycles rather than with other women. Evidence: shown on average; the shift is small and individual patterns differ. How long: the days before a period, rising again after it, cycle after cycle. More: the menstrual cycle and HRV.

Travel and shift work. Night shifts and long flights move your sleep and your body clock. In a small laboratory trial of simulated night work, vagal HRV markers such as RMSSD fell in the group that also ate at night [S19]; it is one laboratory trial and has not been tested in real shift workers. Studies of real round-the-clock shifts are inconsistent [S20], and in one study, people with many years of night work had HRV similar to matched day workers on their days off [S21]. We found no study that measured HRV during real jet lag. What has been measured after flights is sleep: in a large analysis of trips by Oura Ring users, sleep length came back to within about 12 minutes of usual after about two days, but sleep timing had not returned to its usual pattern after 15 days [S22]. That analysis used Oura's own user data; one author works for Oura and another sits on its medical advisory board. Evidence: mixed — a drop with simulated night work in the laboratory, inconsistent results for real shifts, a lasting drop after years of night work not shown, and no data on HRV during jet lag. How long: for HRV, not known; for sleep after a flight, see above.

Dehydration and heat. In one small randomized crossover in young resistance-trained men, high-frequency HRV in the hours after a session done dehydrated was lower than after the same session done hydrated [S23]. It is a single small study in an exercise setting.

We found no study of everyday mild dehydration and a watch's HRV reading, and no data showing that drinking more water raises HRV. For heat, a systematic review of controlled studies in healthy adults concluded that whole-body heat stress may increase sympathetic and reduce vagal influence on the heart [S24]. Regular sauna is a separate question, covered on sauna and heat exposure. These studies measured HRV during or around heating sessions, not a watch's reading the next morning after a hot day or a warm night. Evidence: dehydration — shown in one small study after exercise, no data for everyday life; acute whole-body heat — shown, measured during or around heating, not the next morning. How long: in the dehydration study, the hours after the session; for heat, not known.

Altitude. In the first days at high altitude, HRV usually falls. A meta-analysis of 15 studies with 698 participants included healthy adults who went up to 2,500 m or higher and were measured within their first seven days there: SDNN, RMSSD and related measures were lower than at sea level [S25]. A lower number at altitude is expected. Whether HRV can warn of altitude sickness is contested [S26], so symptoms, not the watch, decide when to stop climbing. Evidence: shown for the first days. How long: at least the first days at altitude; longer stays have been studied much less. More: altitude and HRV.

Smoking. In a large population study, current smokers' HRV was lower the more heavily they smoked, while former smokers had higher HRV than people who had never smoked [S27]. In a study that followed smokers through ordinary days, high-frequency HRV was lower when they had recently smoked than when they had not [S28]. Both are single observational studies. Evidence: shown in two single observational studies (association). How long: not measured; lower high-frequency HRV was seen around recent smoking [S28], and HRV was lower the more someone smoked [S27].

Caffeine. In people who drink coffee regularly, moderate amounts have not been shown to change resting HRV. In a small controlled trial of young, healthy men who regularly consume caffeine, a modest dose of caffeine did not change HRV in the period measured after it [S29], and in a large cohort, the lower HRV of coffee drinkers disappeared after adjustment for other factors [S30]. Caffeine can still reach night-time HRV indirectly: caffeine taken later in the day can shorten or lighten sleep [S31], which brings you back to the sleep factor above. Evidence: not shown in regular drinkers at moderate doses; this does not cover very large doses, energy drinks or people who rarely drink coffee. More: caffeine and HRV.

Age. On average, HRV tends to fall with age [S32]. Age explains differences between people and between decades, not a drop from one week to the next. Healthy people of the same age differ enormously in HRV [S1], and a lower number than someone else's does not necessarily mean poorer health [S2], so compare your HRV with your own baseline rather than with an age table. Evidence: shown at group level. How long: decades; age is context, not the reason for this week's low reading. More: your HRV baseline.

Late eating — mixed: in one small trial, a larger dinner was followed by a lower high-frequency share of HRV through the night [S33], while in another small study a late high-calorie meal did not change a short morning HRV recording [S34]; see why HRV changes from day to day.

When is low HRV a reason to pay attention?

A single low night is usually not a cause for concern. A single low HRV reading does not by itself mean you are stressed or unwell. If your HRV is clearly below your usual range and your resting heart rate is higher for several days in a row, and you feel unwell, treat it as a non-specific signal that the body is under strain. It is a reason to rest and to watch how you feel, not a diagnosis. If you have symptoms, see a doctor rather than relying on your watch.

Whatever your watch shows, get emergency care for: chest pain or pressure; fainting or near-fainting; severe shortness of breath; fast, strong or irregular heartbeat that does not settle at rest (including with dizziness); sudden confusion, weakness on one side, trouble speaking.

A normal-looking reading does not rule out illness either: in the smartwatch study above, not every infected person received an alert [S8]. How resting heart rate is measured and what moves it is covered on resting heart rate.

What can you do?

Start with what you can see. If the night was short, you drank or you trained hard, the low reading already has a likely explanation: remove what you can, give your body time to recover, and look at the trend over the next few days rather than at the next morning alone. What to do next, step by step, is on what to do after a low HRV reading.

Slow breathing is something you can do at any time. Slow breathing is associated with higher vagally mediated HRV; whether a longer exhale adds anything beyond slowing the breath is still debated. In a large meta-analysis, the rise was seen during slow breathing, immediately after a session and after programmes of several sessions [S35]. That is a change during and after the practice; it does not remove whatever lowered your reading, and it is no test of recovery. The wider evidence is on slow breathing, and the techniques that have been compared directly are on breathing techniques compared.

What does the evidence show?

By evidence status.

• Shown. Alcohol [S5] [S6], illness as a non-specific signal [S7] [S8], acute stress [S10], a hard aerobic session [S14], the first days at high altitude [S25], acute whole-body heat during or around heating [S24] and age at group level [S32]. Sleep loss lowers RMSSD, but the measured drop is small [S4]. Dehydration lowered HRV in one small study after exercise [S23]. Smoking went with lower HRV in two single observational studies (association) [S27] [S28]. Before a period there is a small average dip at group level, and individual patterns differ [S16] [S17] [S18].
• Mixed. Long-term stress [S12] [S13]; travel and shift work [S19] [S20] [S21]; late eating [S33] [S34].
• Not shown. A change in resting HRV from moderate caffeine in regular coffee drinkers [S29] [S30]; a lasting drop in resting HRV from work stress [S13] or from years of night work [S21].
• No data. HRV during real jet lag; everyday mild dehydration and a watch's HRV reading; drinking water as a way to raise HRV; how long a dip lasts after an ordinary short night or an ordinary evening of drinking.

What it does not tell you

• A low reading is not a diagnosis. A single low HRV reading does not by itself mean you are stressed or unwell.
• It does not name its cause. Infection, stress, alcohol and travel can produce the same change [S8].
• Causes stack. A late evening often brings drinks, a later bedtime and a late meal at once, and the studies above isolate one cause at a time.
• Group findings are not your response. Your own reaction to alcohol, a hard session or the days before a period may be larger, smaller or absent.
• The labels describe the evidence, not the size of the effect for you. A cause marked shown can still be small, as with sleep loss [S4].
• No medical conclusions. None of these studies diagnoses a condition for an individual.

In ONDA

ONDA is built around practice rather than tracking. With an Apple Watch, or a device that syncs heart data to Apple Health, it reads your HRV and resting heart rate from Apple Health [S36] and builds your own baseline over 14 days, reading signals only after at least 7 nights. ONDA compares each night with your own corridor — the average of your recent nights plus or minus one standard deviation — and flags a night only when it is at least 1.5 standard deviations outside and has changed by a minimum amount: resting heart rate up at least 5 bpm, HRV down at least 15%, breathing rate up at least 2 breaths per minute, with at most one signal every two days. The baseline uses the HRV value that Apple Health stores (Apple Health records HRV as SDNN; Apple Watch Series 12 and Ultra 4 on watchOS 27 show two HRV variants — Recovery HRV and Overall HRV — and measure HRV as often as every five minutes). Such a signal is descriptive: it says that a night is outside your own range, not why [S36]. ONDA does not identify which cause lies behind a change. For practice, ONDA offers guided breathing with spoken and visual guidance and no numeric pacer, and ONDA shows your pulse live during a practice, from the iPhone camera or an Apple Watch. ONDA has no study of its own effectiveness, does not diagnose any condition and does not replace a doctor. See what ONDA measures.