Fitness Trackers: What Metrics Actually Matter (and Which to Ignore)

If you've ever glanced down at your Apple Watch, Garmin, Whoop, or Oura ring and felt overwhelmed by all the numbers, you're definitely not alone. These devices spit out data about your sleep, your steps, your heart, and even things you may have never thought about, like heart rate variability or respiratory rate. The truth is, some of these numbers can be really vaulable for your health and fitness, while others are just noise that can distract you. The challenge is knowing which is which.

The good news is that you don't need to obsess over every single chart and metric. By focusing on a few key daily numbers and occasionally checking some long-term ones, you can get the most benefit without falling down the data rabbit hole. And when it comes to training, especially endurance or strength training, there are a couple of metrics that shine if you know how to use them. Let's break it down.

Daily Metrics to Keep an Eye On

Resting Heart Rate:

One of the most useful numbers to check every day is your resting heart rate. This is essentially your body's "idle speed" when you are completely at rest. A lower resting heart rate usually means your heart is more efficient, which is a good sign of fitness. If you notice that your resting heart rate is suddenly a few beats higher than usual, it could be a red flag - but only if it lasts for more than a day or two. A single off day is nothing to worry about. Maybe you had poor sleep, extra caffeine, or just a stressful morning. What really matters is when your resting heart rate stays higher for several days in a row, which suggests that your body is under more sustained stress, whether from training, illness, or lifestyle factors. Over the long term, tracking your resting heart rate shows you how your cardiovascular fitness is developing. As your heart rate gets stronger, your resting heart rate should tend lower over months and years.

Heart Rate Variability:

Another daily number that can be helpful is heart rate variability (HRV). HRV measures the tiny difference in time between each heartbeat, and it reflects how well your nervous system is balancing stress and recovery. A higher HRV generally means your body is more resilient and well-recovered, while a lower HRV suggests you are fatigued or stressed. Like resting heart rate, a single low HRV day is no cause for alarm. Everyone has ups and downs depending on sleep, hydration, or even emotions. The real insight comes when HRV tends lower for several days in a row, which may be a sign that you need more recovery. Looking further ahead, long-term HRV trends are even more powerful: if your HRV gradually rises over months of consistent training, sleep, and recovery, that's a clear signal that your health and fitness are improving.

Sleep Metrics:

Sleep is another daily metric worth tracking, but not in the hyper-detailed way that many trackers present it. You do not need to worry if your device says you had less "deep sleep" one night. These measurements are often guesses. What your watch is good at is figuring out how much sleep you're actually getting. Adults generally need 7-9 hours per night to function well, and while some people do fine on the lower end, most of us thrive closer to 8. If your tracker shows you're only logging 5 or 6 hours a night, that's a sign your recovery is being compromised. The daily check-in matters because you can catch shortfalls as they happen and prioritise getting to bed earlier the next night. One bad night isn't the end of the world, but two or three in a row can build up into real fatigue, decreased performance, and even a higher chance of getting sick. Long term, the average amount of sleep you get each week or month is even more telling. Looking back over weeks, if you consistently hit 7–8 hours, that’s a strong sign you’re giving your body the foundation it needs to recover, adapt, and get fitter. If your average trends lower, that’s a red flag — and it’s not something you can fix with just one or two catch-up nights. Sleep debt compounds, and the long-term average is what really reflects whether you’re supporting your health and training.

Metrics to Check Occasionally

VO2 Max

Some numbers don't need your attention every day, but they are useful for spotting longer-term changes. VO2 max is one of the best examples. Think of it as your body's engine size - how much oxygen you can use during hard exercise. It's strongly linked with performance, health, and even longevity. Your VO2max won't change much from one workout to the next, so there's no point in checking it every day. But if you look at it monthly or quarterly, you can see whether your training is building your aerobic fitness. Over time, if your VO2 max is trending upward, it's a strong sign that your endurance training is working.

Readiness/Recovery scores

Even readiness or recovery scores, which some trackers boil down to a single number, work best when viewed as part of a pattern. A low score on one day isn't necessarily a reason to skip training if you feel good. But if your recovery scores are consistently low for several days and match up with how you feel, it's worth listening. Over time, if your average readiness trends higher, it means your training and lifestyle are supporting recovery, which is far more meaningful than the occasional red or green score.

Sleep Consistency:

While the daily focus is on hours slept, another valuable metric to check occasionally is sleep consistency - basically, are you going to bed and waking up around the same time each day? Your body's circadian rhythm thrives on routine, and research shows that regular sleep and wake times improve recovery, hormonal balance, and even cardiovascular health. An inconsistent schedule - think four nights of midnight bedtimes and then an early wake-up followed by a weekend sleep-in - can throw your system off even if you're technically getting "enough" sleep.

Metrics You Can Ignore

Calories burned:

Not every number on your device deserves your attention. Calories burned is one of the least useful, because trackers simply aren't accurate enough at estimating energy expenditure. It's better to focus on consistency of activity and nutrition habits than chasing calorie numbers.

Sleep Stages:

Detailed sleep stages are another flashy but unreliable metric. While the graphs look interesting, the truth is that consumer trackers can't measure brain activity, so their guesses at REM versus deep sleep are just that - guesses. Similarly, stress scores are often just another way of showing HRV data.

Rep Counting:

And when it comes to rep counting during strength training, trackers often get confused by movement patterns, so it's better to log your weights and reps manually.

Where Trackers Really Shine: Training

The most powerful use of your tracker is during training, especially endurance workouts. Heart rate zones are a simple but incredibly effective way to guide your workouts.

The most important of these zones for long-term health and endurance is Zone 2. This is the zone where your effort feels steady but not crushing. Training in zone 2 improves mitochondria, the little power plants in your cells, making them more efficient. It also helps your body use fat as fuel and supports metabolic health. Most people benefit from at least three hours of Zone 2 per week, split into a few sessions. Over the course of months, consistent Zone 2 training lowers your resting heart rate, improves your HRV, and raises your VO2 max - changes that won't show up in a single workout, but absolutely will in your long-term fitness.

Then there are the harder zones, Zone 4 and Zone 5. These are the "push yourself" zones - short intervals of near-maximum effort where speaking is nearly impossible. Training here increases your VO2 max, which is essentially your fitness ceiling. It makes you faster, stronger, and more capable of sustaining harder efforts. These workouts are tough but effective, and they should be used sparingly. Day to day, your tracker can help you hit the right intensity and monitor recovery between intervals. Over the long term, adding consistent but measured Zone 4/5 training raises your performance ceiling and slows the natural decline of VO2 max with age.

For strength training, trackers are less useful at counting reps or measuring weights, but they still play a role. They can help you monitor consistency - are you showing up for your sessions each week? - and they can give you clues about recovery through HRV and resting heart rate. It those numbers suggest you are run down, it may be smart to lighten your session. During circuit-style or conditioning lifts, your heart rate monitor can also give you an idea of how much cardiovascular stress the workout is creating. Over the long-term, using trackers to manage recovery ensures you can string together weeks, months, and years of consistent training, which is where the real strength gains come from.

The Bottom Line

Your fitness tracker can be a powerful tool when you know how to use it, but the key is focusing on what matters most. Resting heart rate, HRV, and sleep are worth looking at every day, as long as you remember that one or two days off are normal. The value comes when you spot trends over several days in a row and when you step back to see how the numbers are shifting month by month. Occasionally checks of VO2 max, sleep efficiency, or readiness scores give you additional perspective, and training with heart rate zones - especially spending time in Zone 2 and strategically adding Zone 4 or 5 intervals - is where trackers really shine.