Health and fitness awareness has never been more widespread than it is today. People nowadays are so conscious about their bodies' health that early parties (from 6PM to 10PM) have become a thing. So, it comes as no surprise that smartwatches have grown in popularity as everyday health companions.
More recently, smart rings like the Our and Galaxy Ring have also entered the market,giving people a more discreet way to track wellness data. These wearables can monitor everything from heart rate and body temperature to blood oxygen level and sleep quality.
But with so many numbers and charts popping up on our wrists and fingers, it’s easy to feel overwhelmed. In this article, I will try to break down every common health metric, explaining how it works and what it means in simple terms.
Your heart rate can tell you a lot about yourself. | Image credit – Samsung.
Heart rate is probably the metric you are most familiar with. At rest, most adults have a heart beating around 60–100 times per minute, although athletes tend to have a much lower resting heart rate.
We measure heart rate because it’s a core indicator of health and effort. When you exercise or even get anxious, your heart rate rises; when you’re relaxed or sleeping, it drops. This makes heart rate the best way to determine exercise intensity (for example, how fast you are running). Your heart rate is also a great window into your cardiovascular fitness and stress levels.
The underside of the Galaxy Watch 6. | Image credit – PhoneArena
Your smartwatch or ring measures your heart rate via an optical sensor on the underside, which shines light into your skin. This technique is called photoplethysmography (PPG), and it detects tiny changes in blood volume with each heartbeat. In simple terms, every time your heart pumps, more blood flows through the vessels in your wrist or finger, and the sensor’s light absorption changes. The wearable counts these pulses to figure out the beats per minute.
You’ll sometimes notice the sensor glowing green (or red) when you are fidgeting with your device – don’t worry, that’s the light sensor doing its thing.
Most devices also give you a breakdown of heart rate in five zones, and athletes often use these zones as an easy way to maintain a certain level of effort. Additionally, gadgets can detect abnormal heart rates, so for example when your heart rate remains high even though you are sitting, this could indicate stress or illness.
Heart Rate Variability (HRV)
Heart rate variability, or HRV, might sound technical, but it’s simply a measure of the tiny variations in time between your heartbeats. Even if your heart beats at 60 BPM (once per second on average), it is not a mechanical instrument, so it doesn’t beat like clockwork. Sometimes there are slightly more than 1.0 seconds between beats, sometimes slightly less. HRV captures those small timing differences.
But why would you care about these split-second variations? It turns out HRV is controlled by your autonomic nervous system – the balance between your “fight-or-flight” responses and “rest-and-digest” relaxation responses. When you’re relaxed, your heartbeats have more variability between them (a higher HRV). When you’re stressed or exhausted, the time between beats becomes more uniform (lower HRV) because your body is in a tense, ready-for-action state.
If all of that is a bit difficult to understand, just remember that higher HRV generally indicates your body is adaptable and recovering well, whereas consistently low HRV can be a sign of stress, overtraining, or not enough rest.
Wearables measure HRV by analyzing the same pulse data they use for heart rate.
HRV is a sensitive measurement. It has to factor in things like your breathing, posture, the time of day – so many devices report your HRV as an overnight average when you’re most still.
The results of these tests might be presented to you as a number in milliseconds or as part of a stress/readiness score. The key is to compare to your own baseline. If your average HRV is, for example, 50 ms and one morning you see it drop to 30 ms, your body might be under more stress. Maybe you slept poorly, drank alcohol, or you are feeling unwell.
A higher-than-usual HRV, on the other hand, can mean you’re well-rested and recovered. Some elite athletes track HRV to decide whether to train hard or take a recovery day. For everyday folks like me and you, noticing trends in HRV can help you recognize when life’s stresses (or healthy habits) are impacting your body.
Blood Oxygen Saturation (SpO2): How well you are breathing
Blood oxygen reading on Apple Watch Series 6. | Image credit – Apple
Blood oxygen level, which some wearables show as SpO2 for short, is a measure of how much oxygen your blood is carrying as a percentage of its maximum capacity. In other words, it tells you how well your lungs are oxygenating your blood. It's an important indicator of respiratory health, showing how well your body is absorbing oxygen.
Before I continue, though, I have to stress that SpO2 measurements done by your smartwatch or ring are generally considered medically reliable, so you shouldn’t rely on them as a clinical reference.
WatchOS 11 brought Sleep Apnea warnings to the Apple Watch. | Image Credit – Apple
This rough estimation, however, is still useful to athletes or hikers in various situations. For example, at high altitudes where oxygen is thinner it lets them know if their levels drop too low. Other uses for Sp02 is to catch breathing issues like sleep apnea (where breathing briefly stops, lowering blood oxygen).
Your device measures SpO2 using the same basic PPG principle as heart rate, but with a twist: it shines both red and infrared light into your skin and reads the light that bounces back. Oxygenated blood absorbs red and infrared light differently than deoxygenated blood. The sensor can estimate the percentage of oxygen saturation by analyzing the reflection.
If you’ve ever had a doctor clip a little device on your finger, that was a pulse oximeter doing the same thing. Smartwatches and fitness trackers use the same technology – usually, you take a spot measurement by keeping still for 15–30 seconds, or the watch might sample it automatically during sleep.
Sleep Stages: Light, deep, REM, and awake
The Samsung Health app now offers a sleep environment report, personalized sleep time guidance, and a built-in mindfulness tracker.
Images credit — Samsung
One of the most popular features of modern wearables is sleep tracking. If you’ve ever looked at your app after a night’s rest, you’ve probably seen a breakdown of your sleep into stages: typically light sleep, deep sleep, REM sleep, and sometimes awake time. But what do these sleep stages mean?
Thankfully, most devices give you an explanation for each stage, but in a nutshell, when you sleep your brain cycles through different modes:
Light sleep is the first couple of layers of sleep when you drift off and your body starts to relax – it’s relatively easy to be woken from light sleep.
Deep sleep is the “heavy” sleep where your breathing and heart rate are at their lowest and your body is doing a lot of physical recovery and repair. This stage is important for muscle growth, immune system strength, and overall restoration, so it is arguably the most important sleep metric when you are training or if you are sick.
REM sleep stands for Rapid Eye Movement sleep – this is the fascinating stage where most of our vivid dreaming happens. During REM, the brain becomes almost as active as when you’re awake, your eyes dart around under your eyelids (hence the name), and interestingly, your body is largely paralyzed (so you don’t act out dreams). REM is thought to be crucial for memory consolidation, learning, and mood regulation.
Finally, awake time is simply periods when you woke up (even briefly) during the night. You usually don’t remember these moments, even though you were technically awake at the time.
Wearables track these sleep stages to give you a window into your night. How much time you spend in each stage can affect how refreshed you feel in the morning. If you get plenty of deep sleep, you should feel physically rejuvenated; if you lack REM, you might feel a bit groggy or mentally foggy. Also, unusual patterns (like almost no deep sleep) can prompt you to adjust your habits (maybe avoid late-night caffeine or heavy meals) to improve sleep quality.
But how does your smartwatch or ring know what stage you’re in without sticking electrodes on your head like they do in sleep labs? Well, it works with what it’s got, which are the motion and heart rate sensors. Thanks to them, it detects patterns and tries to make an educated guess. Needless to say, that makes wearables much less accurate as a method, but the benefit is that you are doing it from the comfort of your home and with much less hardware strapped onto you.
Sleep Score
While the breakdown of sleep stages is informative, many wearable apps also boil your entire night’s sleep down into a single Sleep Score. This is usually a number on a scale (often 0–100) indicating overall sleep quality. If the sleep stages are the detailed report, the sleep score is the quick summary.
For example, Fitbit and Oura devices will give you a score each morning, where a score in the 80s might be “good” and anything above 90 is “excellent” sleep. The reason companies created sleep scores is because it can be hard for a non-expert to interpret 37 minutes of REM or 2 awakenings – the score digests all that data into one convenient metric.
It provides an easy benchmark that you can track over time and it kind of gamifies sleep a little, which can motivate you to improve bedtime habits – “I want to beat yesterday’s score” type of thing.
As for how these scores are measured, even though each brand has its “secret sauce,” most sleep scores are a combination of a few components. Fitbit’s sleep score, for example, uses your total time asleep, the time you spent in deep and REM sleep (versus light/awake), and your heart rate during sleep (sometimes called a restoration factor).
Some devices even factor in your blood oxygen variation, as big drops in oxygen might lower the score as it hints your sleep was less smooth.
As a user, the real value is in the trend and using it for self-discovery. Maybe you notice that every Friday night your score drops (maybe because you stay up late watching movies on Thursday) – that can nudge you to adjust that habit.
I myself had a problem with entering the light stage of sleep where you relax all of your body, and then I realized that it was because of overstimulation from TV shows and gaming just prior to going to bed. I started giving myself about an hour away from screens before bed and that — alongside more regular physical activity — helped me fall asleep much more easily.
Steps: Basic, but effective
Don't underestimate the power of walking! | Image credit – PhoneArena
If heart rate is the king of exercise metrics, step count is the king of daily activity metrics. It’s one of the simplest ways to measure how active you were for the day.
A common goal that’s been popularized is aiming for 10,000 steps a day (roughly 5 miles). I recently found out that the 10,000 number is actually somewhat made up (it originated from a marketing slogan in Japan in the 1960s), but the general idea is that a higher step count usually means you’ve spent more time being active, which is great for cardiovascular health, weight management, and — often underestimated — mental health.
The way your smart wearable measures steps is through an accelerometer (motion sensor) that continuously senses your movements in all directions. The device’s software looks for the characteristic up-and-down, rhythmic motion pattern of walking. When you walk, your arm swings gently, and your body experiences repetitive, slight impacts as your feet hit the ground.
Using clever algorithms (and the personal info you gave it like your height, which helps estimate stride length), it filters out random movements and counts constant rhythmic motions as steps. If the movement is faster and more intense (and your arm swings in a smaller arc), it may register as running steps.
But while these algorithms have gotten pretty good, they are not foolproof. For instance, if you’re pushing a shopping cart, your wrist might not move much, and it could undercount steps. Or vigorous arm motions while standing in place might add a few “phantom” steps.
I want to emphasize again, that you definitely don’t have to stick to 10,000 steps per day. The best way to use this metric is by giving yourself a goal that sounds easy enough to motivate you to go outside and walk more. It’s also important to take into account whether your job or other daily activities include a lot of walking. If that’s the case, then you are probably already doing pretty good.
Calories Burned: Measuring your energy output
Keeping track of your calorie expenditure is a key metric for fitness and health. | Image credit – PhoneArena
Many wearables display how many calories you’ve burned in a day or during a workout. This metric aims to quantify your energy expenditure – essentially how much fuel your body burns through movement and just staying alive (yes, staying alive burns a lot of calories).
People often pay attention to this number for weight management and to gauge how intense or effective their workouts were. It’s also satisfying to see a tangible number that represents your hard work after exercise – “I burned 1200 calories on that run!” But interpreting calorie burn needs some nuance because it’s an estimate, not a direct measurement, especially when it comes to smartwatches and smart rings.
Wearables combine several key pieces of information to determine your burned calories:
Your basal metabolic rate (BMR): or the number of calories your body needs to accomplish its most basic life-sustaining functions. Fun fact – BMR usually includes the majority of your burned calories.
Movement and often heart rate: also known as the “active calories” you burn during exercise or just simple movement.
When you set up your device, you usually input age, sex, weight, height – all of which help calculate your BMR. On top of that baseline, the device adds calories for your physical activities. Using the accelerometer, it knows when you walk, run, or are active, and using heart rate data it can gauge intensity. Different activities burn calories differently, and the watch doesn’t truly “know” how many you burn, but it uses formulas to guess based on your movement speed, duration, and heart rate relative to your fitness profile.
Keep in mind that it is crucial that you regularly update your weight, as it is one of the key metrics that your smart wearable uses to give you accurate measurements!
Again, it’s important to remember that there’s always a margin of error. Some studies have even found wrist devices can have an error range off by 10-20% or more!
Activity Rings and Scores: Gamifying your goals
Gamifying your health goals can be an easy "cheat" to achieve them! | Image credit – PhoneArena
We might think ourselves high and mighty, but the truth is that our brains have not changed too much since the caveman days. There’s a reason games can be addictive (think World of Warcraft, for example) – it’s because they give you a constant sense of achieving a goal.
But we’ve learned how to use this trick for more productive things, like our fitness and health. Apple’s Activity Rings are a clear example of gamification being used, and successfully at that! One of the main reasons I am still holding onto my Apple Watch instead of swapping it for a more serious fitness wearable like a Garmin are exactly those rings, and the fact that I can share this information with my friends who also have an Apple Watch.
Of course, what makes a gamified fitness metric even more fun is competition. The ability to compete with someone you know, even when you are not physically next to each other, makes the activity a social exercise, which means you have another strong motivator to get you to move!
Respiratory Rate: Breaths per minute
It not all about the heart.
Respiratory rate is the number of breaths you take per minute. If you inhale and exhale 15 times in one minute while at rest, your respiratory rate is 15. Normally, healthy adults have a resting respiratory rate somewhere between about 12 and 20 breaths per minute. It’s something we typically don’t notice unless we’re winded or meditating, but it’s a fundamental vital sign.
Wearables have started tracking respiratory rate, mostly during sleep, because changes in your breathing rate can offer insights into your health and stress levels.
For example, when you’re anxious or your body is fighting an infection, your breathing might speed up even at rest. During sleep, your respiratory rate is quite regular, so a deviation from your baseline could indicate something’s up (for example, an oncoming illness like a respiratory infection might raise your nighttime breathing rate).
For fitness folks, tracking respiration can also help indicate recovery; some devices use a higher-than-normal respiratory rate as one factor in suggesting you need rest. And of course, conditions like sleep apnea involve abnormal breathing patterns, and some wearables already have the ability to detect it.
As for how your watch or ring can measure your breathing by analyzing your heart’s rhythm. There’s this thing called respiratory sinus arrhythmia: when you breathe in, your heart rate slightly speeds up; when you breathe out, it slows down (that’s why snipers breathe out before a shot). This is perfectly normal – it’s your body coordinating heart and lung function. By detecting this subtle speed-up/slow-down pattern in your heart rate data (via the PPG sensor), the device can count how many breaths you take per minute.
When you check your app, you might see that your respiratory rate was, say, 14 breaths/min last night. If you hover around 14 normally and last night was 14 – all is A-OK. If you jump to 17-18 one night, you might have been congested or had a nightmare or some stress. If it stays elevated night after night, it could hint at something like a developing cold, fever, or other strain on your system.
Skin Temperature Trends: Your inner thermometer
Samsung's skin temperature feature. | Image credit – Samsung
Some newer smartwatches and rings track your skin temperature or the variation in your body temperature over time, although this metric is still not reliable enough.
You can use this metric to see how much your skin temperature deviates from your recent baseline (e.g. “+0.5°C higher than usual” during last night).
Our body temperature fluctuates with our circadian rhythm – it tends to be lower in the early morning hours and higher in late afternoon. But beyond daily swings, changes in baseline can indicate things like fever or hormonal changes. Wearables that track temperature can alert you to unusual spikes that might suggest you’re getting sick before you feel symptoms. For example, the Oura Ring’s temperature data has been known to give a heads-up on flu, as users saw higher-than-normal readings a day or two before other symptoms hit.
Also, tracking temperature trends can help with menstrual cycle awareness – some devices use it to predict fertile windows or period start, since there’s often a noticeable shift in temperature during the cycle. Even lifestyle factors like alcohol consumption or travel (jet lag) can show up as temperature deviations.
However, this is one more metric for which it is important to mention that wearables are not medical instruments. If you truly feel feverish or need an accurate reading, use a proper thermometer.
Stress Level and Readiness Scores
The Galaxy Watch has an Energy Score that represents the daily measurement of your physical and mental readiness. | Image credit PhoneArena
Like I mentioned earlier when talking about heart rate and sleep stages, wearables try to combine multiple metrics into a simpler number, such as a stress score or readiness or recovery score. These are immensely popular in higher-end fitness trackers and smart rings because they attempt to answer: “How is my body doing overall today?” That said, there are multiple memes online of how these scores can sometimes show you the exact opposite of how you are feeling that day. So, while they can serve as a neat way to figure out if you need a rest, it’s best to trust how you feel in real life instead of some numbers that came out of a formula.
Recovery Time and Body Battery
In addition to daily readiness scores, some fitness devices give more specific guidance on recovery. For instance, after you finish a workout, certain sport watches estimate a recovery time – for example, you may see that you need to recover for 36 hours after a moderately hard run, while the recovery after a marathon may be days. So this number is based on the intensity and duration of your exercise (using heart rate, training load, etc.).
It’s a useful guideline to avoid piling on intense workouts without adequate rest.
You might also see something called “Body Battery” – a feature introduced by Garmin, which is a 0 to 100 score of your body’s “energy” at any given time. It’s a lot like a phone battery, but for you. It continuously goes down when you’re active or stressed, and charges up when you rest or sleep. It uses a combination of your heart rate variability, stress, sleep, and activity data to compute this.
Conclusion: Turn data into better decisions
From heartbeats and breaths to movement and sleep, our smartwatches and rings give us a wide variety of information about our bodies. Even though we still can’t trust smart wearables as much as professional equipment, they are capable tools that can help us make positive changes to our lives.
Hopefully, this article has helped you better understand these numbers and charts.
The most important thing to remember, though, is that the best instrument you will always have is your own intuition and feeling. You are your best coach, so coach yourself with your health and best intentions in mind.
Aleksandar is a tech enthusiast with a broad range of interests, from smartphones to space exploration. His curiosity extends to hands-on DIY experiments with his gadgets, and he enjoys switching between different brands to experience the latest innovations. Prior to joining PhoneArena, Aleksandar worked on the Google Art Project, digitizing valuable artworks and gaining diverse perspectives on technology. When he's not immersed in tech, Aleksandar is an outdoorsman who enjoys mountain hikes, wildlife photography, and nature conservation. His interests also extend to martial arts, running, and snowboarding, reflecting his dynamic approach to life and technology.
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