Two kinds of things move your HRV. Age and genes set the range it usually sits in. Inside that range, alcohol, illness, short sleep, hard training, stress and the menstrual cycle push it up and down from day to day.
Welcome to Level 3. In Level 2 you built a 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 and learned to read trends. This level is about using HRV well: knowing what moves it, using it for training and stress, and spotting claims that go too far.
Range-setters and day-movers
A 2024 review of the research sorts the influences on HRV into groups. Some you cannot change, such as age and sex. Others are lifestyle habits, illnesses and outside conditions (Sammito and colleagues, 2024).
It helps to think of two layers:
- Range-setters decide where your baseline sits. Age is the biggest. HRV falls steadily through adult life. Genes matter too, and so does long-term fitness.
- Day-movers push you around inside that range for a night or a few days. These are what you see in your daily readings.
You cannot train your way out of your age. That is one reason lesson 7 warned against chasing someone else’s number. In a sample of 28,175 people, age, sex, body mass index and activity level together explained only a small part of the differences in HRV between people (Altini and Plews, 2021).
How big are the daily effects?
The best single picture comes from that same study. Altini and Plews looked at about 9 million morning readings over five years and compared each person with their own usual values. On average, HRV changed by:
| Factor | Average change in morning HRV |
|---|---|
| High alcohol intake | about 12% lower |
| Being sick | about 10% lower |
| A training day | about 3 to 5% |
| Second half of the menstrual cycle | about 3% |
Remember from lesson 10 that ordinary day-to-day variation is often around 12%. So only the larger effects stand out on a single morning. Smaller ones show up in a 7-day average or across many tagged days.
These are averages across many people who chose to use an app. Your own response can be larger or smaller. The study’s lead author also develops the app the data came from.
The main day-movers
Alcohol
Alcohol gives the clearest and most reliable drop. In 4,098 Finnish employees, each person’s nights after drinking were compared with their own nights without alcohol. An HRV-based recovery score fell more as the dose rose, and even about one drink showed up (Pietilä and colleagues, 2018). Read alcohol and HRV.
Illness
Infections tend to lower HRV and raise 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 , sometimes before you feel ill. HRV cannot tell you what illness you have. Read HRV when you’re sick.
Sleep
Short sleep tends to lower 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 . A 2025 meta-analysis of 11 trials with 549 people found that sleep deprivation lowered RMSSD, though 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). Read HRV and sleep.
Training
A single aerobic session lowers HRV for a while. Full recovery took up to 24 hours after easy sessions, 24 to 48 hours after moderately hard ones and at least 48 hours after hard ones. Fitter people recovered faster (Stanley and colleagues, 2013). Lesson 12 covers training in depth.
Stress
Mental stress tends to lower HRV while it lasts. It often travels with poor sleep, less exercise and more alcohol, which makes it hard to separate. Lesson 13 covers stress and breathing.
The menstrual cycle
A meta-analysis of 37 studies found that 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 tends to fall from the first half of the cycle to the second, with the lowest values just before a period (Schmalenberger and colleagues, 2019). A lower reading in the second half may simply be your normal pattern. Read HRV and the menstrual cycle.
Others
Caffeine has mixed evidence. Heat, altitude, smoking and some medicines can also move HRV. The full list is on what affects HRV.
Sensitive, not specific
Altini and Plews (2021) called HRV a sensitive but not specific marker of stress. It reacts to many kinds of load but cannot tell you which one.
A dip after a party, a dip from a cold and a dip after intervals look the same in your app. Two things help you tell them apart:
- Resting heart rate. After alcohol or during illness, resting heart rate often rises by several percent as HRV falls. After training, the change in heart rate is usually small. See HRV vs resting heart rate.
- Context. Your tags, your symptoms and your diary tell you what the number cannot.
Find out what moves your HRV
You can run a simple experiment on yourself:
- Keep your measuring routine fixed. Same time, same position, same device. Otherwise you are measuring your routine, not the factor.
- Pick one factor. For example, alcohol, late meals or late screens.
- Tag it for two to four weeks. Note every day it happens.
- Compare averages. Average the mornings after tagged days and the mornings after untagged days. A gap bigger than your normal day-to-day spread is worth taking seriously.
- Change one thing at a time. If you change sleep, alcohol and training in the same month, you will not know which one helped.
Go deeper Why one-person experiments are tricky
Factors cluster. Drinking days are often also late nights, rich meals and social stress. So a gap you find may belong to the whole evening, not the drink alone. You can still act on it: the evening as a whole is what you would change.
Expect some noise even with good habits. With a day-to-day spread of around 12%, you need many tagged days before a small effect, such as 3%, stands out.
When it is not a lifestyle factor
Most dips last a night or a few days and have a clear cause. If your 7-day average stays well below your range for weeks with no clear reason, talk to a doctor. And if you have symptoms such as chest pain, fainting, palpitations or breathlessness, get medical help whatever your HRV says.
Next, in HRV for training and recovery, you will use these ideas to plan hard and easy days.