How Smartwatches *Actually* Work: A Mentor's Guide to Your Heart Rate, Sleep, and Wellness Tech

Let’s be honest: modern smartwatches feel a bit like magic.

This little circle of glass and metal on your wrist claims to know when you’re sleeping, how fast your heart is beating, how oxygenated your blood is, and even where you are in your menstrual cycle.

But how? Is it just guessing?

As your enthusiastic mentor in this space, I want to pull back the curtain. This isn’t magic; it’s a fascinating and surprisingly accessible blend of physics, biology, and computer science. Knowing how it works doesn’t just satisfy your curiosity—it empowers you to understand the data and use your device more effectively.

We’re not here to review one specific product, but we’ll use the Sanorum G53, a popular model designed for women, as our case study to see how these technologies come to life.

Ready? Class is in session.

Pillar 1: The “Magic” Green Light (How Your Watch Sees Your Blood)

This is the absolute core of all modern health tracking. When you see features like “Heart Rate Monitor” or “Blood Oxygen (SpO2) Monitor,” they are almost all powered by a technology called Photoplethysmography (PPG).

That’s a mouthful, so let’s call it the “Green Light Trick.”

Here’s the simple breakdown:

1. It’s a Flashlight: On the back of your watch, tiny, bright LEDs flash light into your skin (on the G53, and many others, these are often green).
2. Blood is Red: Your blood is red because it absorbs green light and reflects red light.
3. The “Pulse”: With every heartbeat, a pressure wave (your pulse) pushes more blood through the capillaries in your wrist.
4. The Sensor: A sensor next to the LEDs measures how much green light is being absorbed.

When your heart beats, more blood rushes in, and more green light is absorbed. Between beats, less blood is present, and less green light is absorbed.

By flashing these lights hundreds of times per second, the watch creates a precise graph of these peaks and valleys, and voilà—it counts the peaks to get your Heart Rate (BPM).

What about Blood Oxygen (SpO2)?

This uses the same principle but is one step cleverer. Instead of just green light, the watch uses red and infrared light.

• Oxygen-rich blood (bright red) absorbs different amounts of red and infrared light than…
• Oxygen-poor blood (darker red).

By measuring the ratio of the two different light types being absorbed, the watch’s algorithm can estimate your blood oxygen saturation percentage. It’s an incredible feat of non-invasive engineering, all happening on your wrist.

Pillar 2: The “Jiggle” Detector (How Your Watch Knows You’re Moving… or Sleeping)

The second key sensor in your watch is one you already have in your phone: the accelerometer.

This is simply a tiny, low-power sensor that detects motion, vibration, and orientation. It’s the same technology that knows when to flip your phone’s screen from portrait to landscape.

In a smartwatch, this sensor is a non-stop data-gathering machine.

• For Step Counting (Pedometer): The accelerometer looks for the unique, rhythmic “jiggle” pattern of a human footstep and counts them.
• For Sedentary Reminders: If the accelerometer doesn’t detect any significant motion for a set period (like an hour), it sends a “Time to stand up!” reminder.
• For 70+ Sport Modes: When you tell your Sanorum G53 you’re “Cycling” or “Rowing,” it tells the accelerometer to look for a different motion pattern than “Running.”

The Real Secret: How it Tracks Sleep

This is where the accelerometer teams up with the PPG sensor.

1. “Am I Asleep?” (The Motion): The accelerometer first determines when you fell asleep by detecting a long period of stillness.
2. “How Am I Sleeping?” (The Motion): Throughout the night, it tracks your “jiggle.” Tossing and turning = “Restless” or “Light Sleep.” Long periods of total stillness = “Deep Sleep.”
3. The “Confirmation” (The Heart): The PPG sensor adds another layer. During deep sleep, your heart rate typically slows and becomes very regular. During REM sleep (when you dream), your heart rate can become faster and more variable.

By combining the motion data (accelerometer) with the heart data (PPG), the watch’s software makes an educated, and often very accurate, guess about your sleep stages.

Pillar 3: The “Brain” (How Algorithms Create Meaning)

This is the final, and most important, piece of the puzzle. The sensors—the PPG and the accelerometer—are just data collectors. By themselves, they are “dumb.”

The real magic is in the software algorithms (what the Sanorum G53 marketing calls its “AI smart chip”).

This software is what looks at the raw data—“blip, blip, blip” from the PPG, “jiggle, jiggle, still” from the accelerometer—and translates it into meaningful, human-readable insights.

This is where “Women’s Wellness” features come from.

A feature like Female Cycle Tracking doesn’t use a new sensor. It’s a brilliant piece of software.
1. You provide input (your last period date, cycle length).
2. The Algorithm (the “brain”) uses this data to build a predictive calendar.
3. It gets smarter (sometimes): Advanced trackers can eventually correlate tiny changes in your resting heart rate or body temperature (if the sensor exists) with ovulation, but for most watches in this category, it’s primarily a powerful algorithmic calendar and reminder system.

The same goes for medication reminders, 70+ sports modes, and calorie burn estimates. These are all the “brain” of the watch, processing data from the “senses” (the sensors) to give you a complete picture.

The Practical Science: Connectivity and Staying Power

Finally, a smart device is useless if it’s dead or disconnected.

• Connectivity (Bluetooth): This is the bridge. Your watch uses low-energy Bluetooth to send all its data to your phone’s app for analysis. It also allows for features like Bluetooth Calling—the watch essentially acts as a tiny wireless headset (speaker and mic) for your phone, which stays in your pocket or bag.
• GPS (Via Smartphone): You’ll notice the G53 uses “GPS Via Smartphone.” This is a very common and smart engineering choice. A built-in GPS chip drains a battery extremely fast. By “borrowing” your phone’s GPS signal, the watch gets accurate run/walk tracking without sacrificing its battery life.
• Battery Life & Waterproofing (IP68): A good battery (like the G53’s 7-14 day claim) is crucial. And IP68 is not just a random code. It’s a specific engineering standard:
  • “6” = Totally dust-tight.
  • “8” = Protected from continuous immersion in water (usually ~1.5 meters for 30 mins).
  • What this means: You can wash your hands, get caught in the rain, and even swim (in a pool, not the ocean) without worry.

Your Takeaway: You’re Now an Informed User

See? No magic. Just brilliant, compact science.

That device on your wrist isn’t just a stylish accessory. It’s a miniature lab, constantly gathering data. * The green light (PPG) is “seeing” your pulse. * The motion sensor (accelerometer) is “feeling” your movement. * And the software (“brain”) is translating it all into a story about your health.

By understanding how it works, you can better understand its data and its limitations. You’re no longer just a passive user; you’re an informed partner in your own wellness journey.