As the popularity of smartwatches, also called wearables, increases, so too does the amount of data users can access about their health.
Statistics like resting heart rate, V02 max (maximal oxygen consumption) and heart rate variability (HRV) provide valuable insight into your fitness levels.
But not all measurements are created equal.
While HRV has long been used by elite athletes to dictate their training schedule, it has become a “buzzword” among the broader community.
Matthew Ahmadi, deputy director of the University of Sydney’s Mackenzie Wearables Hub, says HRV has been around for more than 20 years in the competitive sports environment.
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“Over the past 10 years or so, it has really picked up among the general population, mainly due to increases in the use of wearables,” Dr Ahmadi says.
HRV has long been used in the competitive sports environment. (Getty Images: Sportsfile/Sam Barnes)
WHOOP, a leading wearable brand and official partner of the Women’s Tennis Association, has been collecting data on the most consistent factors that boost or harm your HRV.
Global statistics from 2024 show that getting quality sleep, consuming caffeine and having a consistent wake time are most likely to improve your HRV, while consuming alcohol, being sick and highly stressed will see your numbers crash.
So just what does HRV measure, and how might you use it to maximise your workouts?
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HRV indicates how ‘balanced’ your autonomic nervous system is
Dr Ahmadi explains that HRV measures the “variation between heartbeats”.
The heart does not beat consistently (like a metronome or clock).
Dr Ahmadi explains that a high HRV indicates that your autonomic nervous system is in “balance”. (Supplied: Matthew Ahmadi)
So, while your heart might be beating at 80 beats per minute, there will be longer or shorter gaps between each heartbeat.
How variable these gaps are determines your HRV, with a higher number indicating greater variability.
A high number is ideal, and an indication that the body is in homeostasis (or balanced), Dr Ahmadi says.
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This is because HRV is a window into the autonomic nervous system.
The autonomic nervous system consists of two main divisions: the parasympathetic and sympathetic nervous systems.
In simplistic terms, the parasympathetic nervous system is responsible for ‘rest and digest’ processes, while the sympathetic nervous system is responsible for ‘fight or flight’ responses.
Ideally, Dr Ahmadi says, the two are in balance, meaning the body is equally responsive to both inputs.
“The more fit someone is, the more their body is able to adapt to different stressors and therefore recover better,” he says.
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Your HRV score is best understood as a window into the automatic nervous system. (Supplied: WHOOP)
But if your HRV is low, it indicates that one branch of the autonomic nervous system is dominating:
“It generally means your sympathetic system is dominant, so your body is on high alert.”
HRV indicates how ‘ready’ you are to exercise
When your HRV is measured is also critical to its interpretation.
Measuring it before you go to bed will tell you how well the body has recovered from the current day’s activities or stressors, Dr Ahmadi says.
“[Measuring HRV] right after you wake up will give you a good indication of what your body is prepared to handle over the coming day,” he adds.
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The time of day your HRV is read determines how it should be interpreted. (Supplied: WHOOP)
This is the key reason HRV offers an advantage over other measures of fitness.
Not only does a higher HRV indicate a higher level of fitness, it can also be used to dictate what level of intensity you should take on in your current day’s exercise regime.
“In the sport science world, we think of HRV as a proxy for ‘readiness’ [to train],” Kristen Holmes, WHOOP’s principal scientist, says.
Kristen Holmes is WHOOP’s Global Head of Human Performance. (Supplied: Kristen Holmes)
“Higher HRV yields more capacity to train, whereas low HRV means less robustness and less capacity to train.“
WHOOP uses HRV to determine its users’ daily “recovery” scores, grading them as being in the green, yellow or red.
On green days, WHOOP encourages users to go “all out”, while on a red day, it recommends “active recovery” activities like meditation, light walking or yoga.
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Many elite athletes similarly use HRV to adapt their exercise regimes, with research showing those who do so achieve better performance outcomes.
Whoop uses HRV to determine “recovery scores”. (Supplied: WHOOP)
Factors that improve or decrease HRV
WHOOP also collects data on the most common positive and negative influences on HRV.
Positive factors include being well-hydrated, following a healthy diet, getting quality sleep and keeping consistent sleep and wake times.
“That’s what makes it both a great measure and really frustrating,” Dr Holmes says.
“It’s hard sometimes to pinpoint exactly what isn’t allowing me to respond and adapt to external stressors in a functional way.”
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There are, however, consistent factors that lower HRV, with the top being the consumption of alcohol.
In fact, Dr Holmes says alcohol is such an “HRV crusher” that 89 per cent of people who use the platform report decreasing alcohol consumption in the long term.
Across the board, alcohol consumption has been found to lower HRV. (Getty Images: Gregory Lee)
“That’s because people can directly see the performance cost from their behaviours,” Holmes says.
“Your coach can tell you, ‘Hey, alcohol is really bad for your performance’, but until you see directly how it affects your HRV, you’re like … wow, I’m a lesser version of myself today. It’s sobering.”
‘Boosts’ HRV
‘Harms’ HRV
1.
Sleep quality and duration
Alcohol
2.
Caffeine
Fever
3.
Consistent wake-up time
Sleep at altitude
4.
Consistent bed time
High stress zone
5.
Daylight eating
Sickness
Source: WHOOP 2024 Year in Review user data
But you should avoid comparing your HRV scores to others’.
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Generally, the older you get, the lower your HRV will be, while women also tend to have a lower HRV on average.
If you want to improve your HRV, both Dr Ahmadi and Dr Holmes instead advise taking a baseline level and judging any changes relative to your own scores.
Generally, it isn’t useful to compare your HRV score to others’. (Getty Images: Thomas Barwick)
Don’t panic if your HRV is low
It’s also important to note that a low HRV is not always bad.
As one example, Dr Ahmadi says that you should expect to see a drop-off in your HRV if you are beginning a new exercise program, especially if you were previously sedentary.
“The first few weeks, you’re likely to see a big decrease in your HRV on a daily basis,” he says.
“But ideally, if the program is working the way it should be, you should start to see your HRV increase over the coming weeks and months, and then taper off as your body adapts to the current program’s load.”
Once your HRV starts to stabilise, it’s a good indication that you are ready to “progress” your program.
But if you don’t see an improvement in your HRV, it’s likely you need to take your foot off the pedal.
“If you see a big drop-off, then it’s a good idea to not push as hard the following day, because your body is giving you signs that it needs time to recover,” he says.
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“As you settle into a new training regime, your body will adapt and you’ll be able to handle higher stress and workout loads closer together.”
“My thesis focuses on physical activity and physical capacity in older adults, including both healthy older adults and patients with severe hip osteoarthritis. I have examined the reliability of various field-based physical fitness tests, compared physical capacity between healthy older adults and those with hip osteoarthritis, and evaluated how exercise affects physical capacity in older adults and how total hip arthroplasty affects physical capacity in patients with hip osteoarthritis”, says Manne Godhe , PhD student at the Department of Molecular Medicine and Surgery .
Which are the most important results?
“The most important results show that field-based physical fitness tests have generally good reliability for older adults and can be used in both research and clinical practice. Structured exercise programs of just eight weeks (twice weekly) provide significant improvements in muscular endurance, strength, cardiorespiratory fitness, and motor fitness in older adults”.
“Another key finding is that severe hip osteoarthritis significantly impairs physical function and activity levels compared to healthy older adults. Total hip arthroplasty leads to substantial improvements in both physical fitness and activity patterns. One year after surgery, patients achieved international physical activity recommendations”.
How can this new knowledge contribute to the improvement of people’s health?
“This knowledge can help promote exercise for older adults in community care, recreational activities, and healthcare. Simple, cost-effective field tests enable better evaluation of physical functions in older adults and allow monitoring of changes over time. For end-stage hip osteoarthritis patients, this knowledge can improve rehabilitation strategies and set realistic recovery expectations after surgery”.
“The results also emphasize the importance of regular physical activity for maintaining health and function in older adults and demonstrate that even short-term exercise interventions can provide meaningful benefits for this population”.
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What are your future ambitions?
“In the future, I want to continue research on how exercise can be optimized for different groups of older adults. I also want to develop and validate more field-based tests that can be implemented in clinical practice and preventive healthcare”.
Dissertation
Friday May 23, 2025 at 09:00, GIH, Lidingövägen 1
Thesis
Physical Activity and Fitness Measurements in Healthy Older Adults and Osteoarthrities Patients Undergoing Total Hip Arthroplasty
Need a good night’s sleep? Cut back on exercising in the evening. Getty Images
If you’d like to sleep well tonight, you should probably avoid exercising this evening, especially if your workout will be intense.
That’s the takeaway from a new study of almost 15,000 active men and women. It found that exercising within about four hours of bedtime makes it harder to fall asleep and reduces how long you spend slumbering by as much as 43 minutes.
The effects were most pronounced when workouts were long, intense or both, but almost any evening exercise influenced how well people slept.
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“I do my best now to avoid exercising late in the evening,” said Josh Leota, a researcher at Monash University in Australia, who led the new study.
But there may be ways to minimize the effects if evening happens to be the only time you can — or care to — work out.
The link between exercise and sleep
For decades, researchers have been puzzled by the relationship between sleep and exercise. According to most past research, active people sleep better than the sedentary, but not always. Some studies suggest morning workouts improve sleep, while later workouts don’t, but others seem to show any movement, at any time, helps people nod off earlier.
Most of these studies have been quite small, though, often involving fewer than 20 volunteers, and relied on people’s memories of when and how they worked out and snoozed.
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So for the new study, published in April in Nature Communications, researchers at Monash teamed up with the activity-tracker maker Whoop to parse anonymized data from 14,689 men and women aged 18 to 87 who’d worn a Whoop tracker for at least a year. (Whoop provided access to the data but “did not have any input into the analysis or results,” Leota said.)
The records included extensive details about when and how intensely people exercised every day, based on their heart rates, and also how well they’d slept that night, including when they’d nodded off, how long they’d remained asleep and the overall quality of their slumber.
36 extra minutes to fall asleep
The researchers were interested in how late-day exercise changes sleep — since previous studies had so often disagreed with one another. They first categorized people’s workouts as light, moderate, hard or maximal, corresponding, in broad terms, to a brisk walk, easy jog, long run or prolonged high-intensity interval training. They also took note of when people worked out and mapped their sleep.
Then they cross-checked. Did people sleep better or worse after they worked out close to bedtime? What if the exercise was gentle? What if they pushed themselves?
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The answers consistently showed that “later exercise timing and higher exercise strain” were each strongly linked to worse sleep, the scientists wrote in the study. Even relatively modest evening workouts, such as light weight training or a gentle gym class, could somewhat disrupt sleep.
But the impacts intensified along with the intensity. If people ran an after-hours half-marathon or played a rousing late-night soccer, hockey or basketball game within about two hours of their usual bedtime, they needed an average of 36 extra minutes to fall asleep.
Finish that same strenuous exercise even later at night, after someone’s usual bedtime by an hour or two, and he or she would need an extra 80 minutes to doze off.
People also slept less, in total, after hard, evening exercise, and the quality of their sleep declined, with frequent waking, tossing and turning.
How to wind down after a late workout
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The researchers didn’t look at why this happens, but they suspect people were too wound up, physiologically. Participants’ tracker data showed their heart rates were still elevated hours after strenuous evening exercise, while, at the same time, their heart rate variability, which should be somewhat high, remained stubbornly low.
In essence, Leota, said, people got too pumped up by vigorous, late-night workouts to easily drift off or stay asleep. “A basic rule of thumb,” he said, “is the harder you work out, the more time you need to give yourself to recover before going to sleep.”
If you do need to exercise late in the evening, you might want to try meditation, gentle yoga or other relaxation techniques afterward to calm your revved-up body, Leota said.
Even better, “if you can exercise earlier in the day, that would be preferable,” he said.
But if the evening is your best option, stick with it. “We are definitely not discouraging exercise,” Leota said. “For the vast majority of people, any exercise is better than no exercise. We would just recommend trying to finish as early as possible or opting for lighter workouts.”
For anyone who menstruates, you’ll know that your hormones can affect how you feel at different times in your cycle. However, did you know these hormones can also impact your athletic performance? If you didn’t, you’re not alone — a 2019 study conducted by researchers at St Mary’s University in Twickenham, England, analyzed more than 14,000 female Strava members. 72 percent of women said they have never received any education regarding exercise and their menstrual cycle.
Women’s Health Week
This article is part of Tom’s Guide’s Women’s Health Week — a series of content that explores how technology and the right workouts can support and empower women through every phase of life.
If you have a ‘normal’ menstrual cycle — the monthly process where the rise and fall of certain hormones prepares your body for a possible pregnancy — then you will have, on average, 450 periods throughout a lifetime. So, it makes sense to understand what’s happening in your body and how movement can help. Research has continually found that exercise can help to beat a bad mood and even boost dwindling energy levels.
