HOW WEARABLE DEVICES TRACK YOUR HEALTH?

Nowadays, almost everyone wears a smartwatch or fitness band to check their steps, heart rate, sleep, and oxygen level. But have you ever wondered how this tiny wearable device actually gives you information about your body? 

What seems like a simple screen with a few blinking lights is actually a powerful blend of miniature sensors, clever physics, biomedical engineering, and smart software algorithms, all packed into something light enough to sit on your wrist. Think of your wearable as a mini health laboratory that never sleeps. 

It measures signals that even doctors once needed bulky machines to capture, and it turns those signals into easy-to-read data that helps you better understand your body. 

Whether it’s telling you how fast your heart is beating, how deeply you slept last night, or how much oxygen your blood is carrying, your device is constantly analyzing, calculating, and learning all in real time. 

Let’s dive into some of the examples to understand the science behind these devices better. 

Smartwatch as a health tracking device. Image Source:  Pixabay

Heart Rate - Your Watch Measures Your Pulse With Light

Most wearable devices track the heart rate by using green light on the back. When your heart pumps, blood rushes through your wrist, absorbing some of that green light. 

Heart-rate monitoring in wearables uses a clever optical method called Photoplethysmography (PPG). That tiny green light on the back of your smartwatch isn’t just for show; it's shining into your skin because your blood absorbs green light really well. 

Each time your heart beats, the amount of blood in your wrist briefly increases, absorbing more of that light. Between beats, less blood is present, so more light bounces back.

A small sensor called a photodiode catches this returning light and turns it into a smooth signal that rises and falls with every heartbeat. Your watch then uses simple algorithms to read these tiny changes and calculate your heart rate. 

In the simplest sense, your device is constantly measuring how much light your blood absorbs to “count” your heartbeats. It’s like having a mini science experiment happening on your wrist all the time that is quiet, accurate, and surprisingly smart. This means your watch is literally counting your heartbeats with the help of light. 

1. How Does Your Watch Know Every Step You Take?

   
   

Step counts are tracked by a small device called an accelerometer. Think of it as a miniature motion detector that constantly feels how your wrist moves throughout the day. 

When you walk, your arms swing in a steady, repetitive pattern, and the accelerometer picks up these rhythmic changes in motion. Your watch then uses this pattern to recognize, “Yes, that’s a step!”

 If you start running, the motion becomes faster and more forceful, and the device adjusts its calculations accordingly. When you sit still, the sensor remains quiet because there’s no meaningful movement to record. Of course, it’s not perfect, as vigorous dancing or waving your hands around might trick it into counting a few bonus steps. 

Overall, this little sensor does an impressive job of turning everyday motions into useful activity data, helping you understand how active you truly are.

2. How Can Your Watch Measure Oxygen in Your Blood Using Light?

   
   

Measuring blood oxygen level, or SpO₂, might seem like something that only hospital machines can do, but modern wearables accomplish it using a surprisingly simple idea: light. 

Because oxygen-rich and oxygen-poor blood absorb these wavelengths differently, your device shines two types of light—red and infrared—into your skin. It is comparable to shining a flashlight through two distinct liquids and observing how the brightness varies according to the liquid's composition. 

This light reacts with the blood passing through your tiny blood vessels when it penetrates your skin. Depending on the amount of oxygen in your haemoglobin, some of the light is absorbed, and the remainder bounces back toward the watch. 

This returning light is captured by a tiny photosensor, which calculates the amount of each color that returned. The watch can determine your oxygen saturation with amazing accuracy by measuring the absorption of red and infrared light. 

This information is particularly helpful while you're exercising, sleeping, or when your breathing fluctuates.

A photo of the SmartCare wrist-worn pulse oximeter. Source: Study 

3. Sleep Tracking: Your Watch Watches You Sleep (In a Good Way!)

   
   

One of the most intriguing characteristics of contemporary wearables is sleep monitoring, which allows your gadget to keep an eye on your body even while you are totally unconscious of it. 

It can assess your sleep stages using a variety of tiny signals your body sends out, even if it cannot read your dreams. One of the primary indicators is movement. For example, your wearable detects light sleep if you toss, turn, or move around a lot, but it detects deeper, more restorative sleep if you stay motionless for extended periods of time. 

Your heart rate also plays a major role: during deep sleep, your heart slows down and becomes steady, while during lighter sleep or dream-filled REM sleep, it becomes more variable. 

Your watch tracks your breathing pattern in addition to these signals because calm, leisurely breathing frequently signifies that your body is completely relaxed. The gadget generates your sleep graph—a summary of how long you slept, how deeply you dozed, and how frequently you woke up—by combining various data using intelligent algorithms (even the times you reached for your phone at midnight).

Image source: SleepFoundation (open educational graphics on sleep stages).

Conclusion

In summary, wearable technology is much more than just cool gadgets; they are like  little science labs that operate silently on your wrist using only movement, light, and a few intelligent sensors. 

They are able to comprehend your oxygen levels, heart rate, activity, and even how well you slept the previous night. What's the best part? Nothing needs to be done by you. Your gadget learns from every step you take, breath you take, and heartbeat, enabling you to take better care of your health. 

Therefore, keep in mind that your watch is not only speculating when it advises you to get up, breathe, or take a nap. It keeps you informed, conscious, and one step closer to a better version of yourself by utilizing actual science.

References

Main article: A.S.Hovan George, Aakifa Shahul, & Dr.A.Shaji George. (2023). Wearable Sensors: A New Way to Track Health and Wellness. Partners Universal International Innovation Journal (PUIIJ), 01(04), 15–34. https://doi.org/10.5281/zenodo.8260879

(Supporting article)

Kaewkannate, K., Kim, S. A comparison of wearable fitness devices. BMC Public Health 16, 433 (2016). https://doi.org/10.1186/s12889-016-3059-0

Kim, K. B., & Baek, H. J. (2023). Photoplethysmography in Wearable Devices: A Comprehensive Review of Technological Advances, Current Challenges, and Future Directions. Electronics, 12(13), 2923. https://doi.org/10.3390/electronics12132923