HRV is affected by things you can’t change, such as age, sex and genes, and by things you can: sleep, alcohol, stress, training, illness, caffeine and, for many women, the menstrual cycle. Alcohol, illness and hard training lower HRV most clearly in the short term, and your own baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline is the best way to see which factors matter for you.
The factors at a glance
| Factor | Usual effect on resting HRV | How fast and how long | Strength of evidence | Read more |
|---|---|---|---|---|
| Sleep loss | Lowers RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD after nights of restricted sleep | Seen after nights of restricted sleep | Moderate | Sleep and HRV |
| Alcohol | Lowers HRV during sleep, more with higher doses | The same night and the next morning | Strong | Alcohol and HRV |
| Stress | Tends to lower vagal (rest-and-digest) activity | During and after a stressful period | Moderate | HRV and stress |
| Training | Lowers HRV after hard sessions, then it recovers | Up to 24 hours after easy sessions; 48 hours or more after hard ones | Moderate | HRV and training |
| Illness | Lowers HRV and raises resting heart rate 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 while you are sick | While you are sick | Moderate | HRV when you’re sick |
| Caffeine | Results differ between studies | Short term | Mixed | Caffeine and HRV |
| Menstrual cycle | Vagal measures tend to be lower in the second half of the cycle | Changes across the month | Strong | HRV and the menstrual cycle |
| Age | HRV falls with age | Over years | Strong | Normal HRV by age |
How we grade the evidence: Strong means a meta-analysis or large studies that compare the same people with themselves. Moderate means several smaller studies, or one large study. Mixed means studies disagree.
Factors you can’t change
Some influences set the range your HRV usually sits in. A 2024 review of the research groups them as factors you cannot influence, such as age, sex and ethnicity, alongside diseases, lifestyle habits and outside conditions (Sammito and colleagues, 2024).
- Age. HRV falls steadily through adult life. In a sample of 28,175 people, age was more strongly linked to HRV than sex, body mass index or activity level (Altini and Plews, 2021).
- Sex. Differences between women and men are small and depend on the measure. In the same sample, HRV was not associated with sex.
- Genes. Twin studies suggest part of your typical HRV is inherited (Sammito and colleagues, 2024).
- Time of day. HRV rises during the night and falls considerably in the morning hours (Sammito and colleagues, 2024). This is one reason readings must be taken at the same time each day.
None of these is something to fix. They are why a 55-year-old and a 25-year-old should not compare numbers. See normal HRV by age.
Sleep
Short sleep tends to lower HRV. A meta-analysis of 11 trials with 549 participants found that sleep deprivation lowered RMSSD, the vagal measure most wearables use, although SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN did not change significantly (Zhang and colleagues, 2025). The studies varied a lot in how they restricted sleep and measured HRV.
Shift work that includes nights also tends to push the nervous system towards its “ fight or flight Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system ” side (Sammito and colleagues, 2024). Since most wearables measure HRV during sleep, a short or broken night also gives the device less clean data to work with. Read more in sleep and HRV.
Alcohol
Alcohol is one of the most reliable ways to lower HRV. In 4,098 Finnish employees, each person’s nights with and without alcohol were compared. Alcohol lowered vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone in the first three hours of sleep, and the effect grew with the dose. An HRV-based recovery measure fell by about 9, 24 and 39 percentage points after low, moderate and high intake (Pietilä and colleagues, 2018). Even low intake, up to 0.25 g of alcohol per kg of body weight, had a measurable effect. Being young or physically active did not protect against it.
In a separate study of morning readings, HRV was about 12% lower after high alcohol intake (Altini and Plews, 2021). See alcohol and HRV and why your HRV is low after drinking.
Stress and mood
Psychological stress tends to lower HRV. A review of 37 studies found that HRV changed in response to stress in most of them, most often as lower parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system (vagal) activity (Kim and colleagues, 2018). Most of these studies measured short bouts of stress set up in a lab, so they say more about minutes than months.
Day-to-day stress is harder to pin down, because it often comes with poor sleep, less exercise or more alcohol. HRV can tell you your system is loaded, but not which of these is the cause. Read HRV and stress and does anxiety lower HRV?, and for a way to calm the system, can breathing exercises raise HRV?
Training and exercise
A hard workout lowers HRV for a while, and then it recovers. A review of studies after aerobic exercise found that full recovery of vagal activity took up to 24 hours after low-intensity sessions, 24 to 48 hours after moderately hard ones, and at least 48 hours after high-intensity ones. Fitter people recovered faster (Stanley and colleagues, 2013).
Over months, regular endurance training usually raises resting HRV (Sammito and colleagues, 2024). So a short-term dip after training is expected, and a rising long-term trend is a good sign. Read HRV and training, why your HRV is low after a workout and should you train when your HRV is low?
Illness and fever
Being sick tends to lower HRV and raise resting heart rate. In a large app-based study, morning HRV was about 10% lower and resting heart rate about 6% higher on days when people reported being sick (Altini and Plews, 2021). HRV alone cannot tell you that you are getting ill. Read HRV when you’re sick and can HRV warn you that you’re getting sick?
Caffeine
The evidence on caffeine is mixed. A systematic review of 13 studies found the best evidence for a small increase in vagal HRV after caffeine, not a decrease. But the studies differed so much in design, dose and measurement that the authors called the overall effect unclear (Koenig and colleagues, 2013). If you drink coffee, the safest approach is to take your morning reading before it. Read caffeine and HRV.
The menstrual cycle
For many women, HRV changes across the menstrual cycle. A meta-analysis of 37 studies with 1,004 people found that vagal activity fell from the first half of the cycle (follicular phase) to the second half (luteal phase). The drop was largest in the days just before a period (Schmalenberger and colleagues, 2019). In app data, the shift between the two halves was about 3% (Altini and Plews, 2021).
This means a lower reading in the second half of your cycle may simply be your normal pattern. Read HRV and the menstrual cycle.
Other things that move HRV
A 2024 review lists several more influences (Sammito and colleagues, 2024):
- Breathing. Breathing rhythm drives part of the beat-to-beat variation. Breathe normally during a reading.
- Heat. High temperatures activate the “fight or flight” side of the nervous system and lower HRV.
- Altitude. Thin air at altitude tends to activate the “fight or flight” side at first and lower HRV over longer stays.
- Smoking. Smoking is linked to lower HRV, more so the more you smoke.
- Medication. Some medicines, including beta-blockers, ACE inhibitors, antiarrhythmics and some psychiatric drugs, can affect HRV. Do not stop or change a medicine because of your HRV. Talk to your doctor.
How big are these effects?
One study gives a rough sense of scale. Altini and Plews (2021) analysed about 9 million morning readings from app users over five years, comparing each person with themselves. Compared with each person’s usual values, HRV changed by:
- about 3 to 5% after training, depending on the analysis
- about 3% between the two halves of the menstrual cycle
- about 10% during sickness
- about 12% after high alcohol intake
These are averages. Your own response may be larger or smaller. In the Finnish alcohol study, the effect was stronger in younger people and in people with a lower usual heart rate during sleep (Pietilä and colleagues, 2018).
How to tell which factor changed your HRV
Your HRV responds to many things at once, so a single low reading rarely has a single cause. Altini and Plews (2021) described HRV as sensitive but not specific: it shows that something is loading your system, not what. A simple method helps:
- Tag your days. Note alcohol, late meals, hard training, poor sleep, illness and, if relevant, your cycle day.
- Look at the trend, not the day. Compare a 7-day average with your usual range.
- Change one thing at a time. If you think alcohol matters for you, skip it for two weeks and watch the trend.
The lesson on building your HRV baseline shows how to set this up. For changing patterns, also see why your HRV changes so much day to day.
What raises HRV over time
The steady habits that help most are the ones above in reverse: regular endurance exercise with enough recovery, enough sleep, less alcohol and, where you can, less ongoing stress. Changes take weeks to months to show. See how to increase your HRV.
When a drop needs a doctor, not a lifestyle fix
Most dips last a night or a few days and have an obvious cause. A drop well below your own baseline that lasts for weeks without a clear reason is worth discussing with a doctor. So are symptoms such as palpitations, fainting, chest pain or breathlessness, whatever your HRV says. Read what a low HRV means.