6 Ways Your Smartwatch Is Lying to You, According to Science

You check your smartwatch after a run. Your fitness score has dropped. You’ve burned hardly any calories. Your recovery score is really low. It’s telling you to take the next 72 hours off exercise. The worst bit? The whole run felt amazing. So why’s your watch telling you the opposite?

it’s because smartwatches and other fitness trackers aren’t always accurate. These devices shape how people think about health and exercise, but they don’t measure most metrics directly. Instead, many common metrics are estimates—and they’re not as accurate as you might think.

1. Calories Burned

Calorie tracking is one of the most popular features on smartwatches. However, the accuracy leaves a lot to be desired. Wearable devices can under- or overestimate energy expenditure (often expressed as calories burned) by more than 20%. These errors also vary between activities. For example, strength training, cycling and high-intensity interval training can lead to even larger errors.

This matters because people often apply these numbers to guide how much they eat. For example, if your watch overestimates calories burned, you might think you demand to eat more food than you really need, which could result in weight gain. Conversely, if your watch underestimates calories burned, it could lead you to under-eat, negatively impacting your exercise performance.

2. Step Counts

Step counts are a great way to measure general physical activity, but wearables don’t capture them perfectly. Smartwatches use accelerometers to detect motion, but they can mistake arm movements for steps—such as when you’re typing, gesturing, or driving. Conversely, they may miss steps if your arms are still while walking, like when pushing a stroller or shopping cart.

Studies have shown that step count accuracy can vary significantly depending on where the device is worn (wrist vs. Hip) and the individual’s gait. While trends over time can still be useful, relying on a single day’s step count for health decisions may be misleading.

3. Sleep Tracking

Many smartwatches claim to track sleep stages—light, deep, and REM sleep—using movement and heart rate data. However, these devices cannot measure brain activity directly, which is the gold standard for sleep staging (polysomnography). Smartwatches often misclassify wakefulness as sleep or confuse light and deep sleep stages.

Research comparing wearable sleep trackers to clinical sleep studies has found moderate agreement for total sleep time but poor accuracy for sleep stage classification. While smartwatches can assist identify broad sleep patterns, they should not be used to diagnose sleep disorders or make clinical decisions.

4. Heart Rate Monitoring

Optical heart rate sensors in smartwatches work by shining light into the skin and measuring blood flow changes. While generally accurate at rest, these sensors can struggle during intense exercise due to motion artifact, poor fit, or skin tone variations. Studies have shown that accuracy decreases during high-intensity interval training, weightlifting, or activities with irregular wrist movement.

For most users, resting and moderate-exercise heart rate readings are reliable enough for general wellness tracking. However, athletes or individuals with heart conditions who need precise data during intense efforts may find chest strap monitors more dependable.

5. Fitness and Readiness Scores

Many smartwatches provide a daily “fitness score” or “readiness score” that claims to tell you how recovered you are and whether you’re ready to train hard. These scores are typically calculated using a combination of heart rate variability, recent activity, sleep, and stress metrics—all of which are themselves estimates.

Because these scores rely on multiple layers of estimation, their accuracy can be compounded by errors in each underlying metric. A low score might suggest you need rest, but if the underlying data is flawed, the recommendation could be misleading. Listening to how your body feels—fatigue, soreness, motivation—often provides more meaningful insight than a single algorithmic score.

6. Stress Tracking

Some smartwatches estimate stress levels using heart rate variability (HRV), which measures the variation in time between heartbeats. Lower HRV is often associated with higher stress. However, HRV is influenced by many factors beyond stress, including fitness level, hydration, caffeine, alcohol, and even breathing patterns.

Because smartwatches interpret HRV through proprietary algorithms that aren’t always transparent, the resulting stress scores can be inconsistent or misleading. A high stress reading might reflect a tough workout rather than psychological stress, or vice versa. As with other metrics, trends over time may be more useful than single-point readings.

What You Can Do

Smartwatches can still be valuable tools for tracking trends and building awareness of your habits. But it’s important to understand their limitations. Instead of treating any single metric as absolute truth, use your device to observe patterns over days and weeks.

Most importantly, pay attention to how you feel, how you perform, and how you recover. Your subjective experience—energy levels, mood, sleep quality, and exercise tolerance—is often a more reliable indicator of your health and fitness than any number on a screen.

As wearable technology continues to improve, future sensors and algorithms may close some of these accuracy gaps. But for now, the smartest approach is to use your smartwatch as a guide—not a guru.