Why Is There a Red Light Underneath My Apple Watch?

AvatarByArmando Lewellen Smartwatches

If you’ve recently noticed a mysterious red light glowing from underneath your Apple Watch, you might be wondering what it means and whether it’s something to be concerned about. This subtle yet distinct glow can catch your attention, especially if you’re unfamiliar with the watch’s inner workings or health-monitoring features. Understanding why this light appears is key to unlocking the full potential of your device and ensuring it functions as intended.

The red light beneath your Apple Watch is more than just a random glow—it plays a crucial role in how the watch interacts with your body. It’s tied to the device’s advanced sensors designed to monitor your health and fitness, offering insights that can help you stay on top of your well-being. While the light might seem unusual at first, it’s actually a sign of the sophisticated technology working behind the scenes.

As you dive deeper into this topic, you’ll discover the purpose of the red light, how it contributes to the Apple Watch’s health tracking capabilities, and what it means for your daily use. Whether you’re a new user or a longtime owner, gaining clarity on this feature will enhance your overall experience and help you make the most of your wearable technology.

How the Apple Watch Uses Red Light Technology

The red light underneath the Apple Watch is part of its advanced sensor system designed for health and fitness tracking. This light is emitted by a series of LEDs that work in conjunction with photodiodes to measure various physiological parameters by interacting with the wearer’s skin and blood vessels.

Primarily, the red and infrared LEDs, along with green LEDs, serve different purposes:

  • Red and Infrared LEDs: Used mainly for measuring blood oxygen levels (SpO2) by detecting how much light is absorbed or reflected by the blood.
  • Green LEDs: Primarily used for heart rate monitoring, as the green light is absorbed by oxygenated blood, allowing the watch to calculate pulse rates.

The sensors employ a technique called photoplethysmography (PPG), which detects blood volume changes in the microvascular bed of tissue. When the red light is emitted, some of it penetrates the skin and is either absorbed or reflected back depending on the blood flow and oxygen saturation. The photodiodes then measure the reflected light intensity to derive meaningful health metrics.

Reasons You Might See the Red Light

Seeing the red light underneath your Apple Watch is completely normal during certain functions, but it can be surprising if noticed unexpectedly. The most common reasons for the red light activation include:

  • Blood Oxygen Measurement: When using the Blood Oxygen app, the red and infrared LEDs illuminate to measure oxygen saturation.
  • Background Health Monitoring: On newer Apple Watch models, the device periodically measures blood oxygen levels in the background, causing the red light to activate intermittently.
  • Workout Tracking: During workouts, the watch might use red light sensors to improve accuracy in heart rate and oxygen level data collection.
  • Sleep Tracking: While asleep, the Apple Watch can periodically check blood oxygen levels, activating the red LEDs briefly.

If the red light appears continuously or outside of these contexts, it may indicate a sensor issue or a software glitch. Ensuring the watch’s software is up to date and performing a restart can help resolve such anomalies.

Impact of Skin Tone and Fit on Red Light Functionality

The effectiveness of the red light sensors can vary based on individual factors such as skin tone and how tightly the watch fits on the wrist. Darker skin tones can absorb more light, potentially affecting the accuracy of the readings. Similarly, a loose fit may allow ambient light to interfere with the sensor’s ability to detect reflected light accurately.

To optimize sensor performance:

  • Wear the watch snugly but comfortably on your wrist.
  • Position the watch just above the wrist bone.
  • Keep the sensor area clean and free from dirt or sweat.
  • Avoid moving the wrist excessively during measurements.

Common Issues and Troubleshooting Tips

If you notice the red light staying on longer than expected or the sensors not working correctly, consider these troubleshooting steps:

  • Restart the Apple Watch: A simple reboot can fix temporary glitches.
  • Update Software: Make sure the watchOS is the latest version to benefit from bug fixes and improvements.
  • Clean the Sensor Area: Use a soft, lint-free cloth to wipe the back of the watch.
  • Adjust Fit: Ensure the watch is worn correctly and snugly.
  • Reset Calibration: In some cases, recalibrating the watch’s health sensors can help improve accuracy.
Issue Potential Cause Troubleshooting Step
Red light stays on continuously Software glitch or sensor malfunction Restart watch, update software
Inaccurate readings Loose fit or dirty sensors Adjust fit, clean sensors
No red light during blood oxygen measurement App or watchOS error Force quit app, restart watch
Intermittent red light activation Background monitoring functioning normally None needed, normal operation

Purpose of the Red Light on the Apple Watch Underside

The red light located on the underside of an Apple Watch primarily serves as part of the device’s optical heart rate sensor system. This sensor uses photoplethysmography (PPG) technology, which involves shining light into the skin and measuring the amount of light that is either absorbed or reflected back to the sensor. The variations in light absorption correspond to changes in blood flow, allowing the watch to estimate heart rate accurately.

Key functions of the red light include:

  • Heart Rate Monitoring: The red light works alongside green LEDs and photodiodes to track your pulse by detecting blood flow changes in your wrist.
  • Blood Oxygen Measurement: On models equipped with a blood oxygen sensor, red and infrared lights are used to assess blood oxygen saturation (SpO2) levels through similar optical techniques.
  • Sleep Tracking and Fitness Metrics: Continuous monitoring enabled by these sensors provides data for sleep analysis and exercise performance metrics.

How the Red Light Functions Within the Sensor Array

Apple Watch models incorporate multiple LEDs and photodiodes arranged strategically under the watch casing. The red light is part of a broader sensor array that switches between different wavelengths of light depending on the measurement being taken.

Light Type Primary Use Functionality
Green LEDs Heart Rate Monitoring Provides high contrast by highlighting blood flow changes due to hemoglobin absorption.
Red LEDs Blood Oxygen Measurement Penetrates deeper into the skin to detect oxygen saturation in blood vessels.
Infrared LEDs Blood Oxygen and Sleep Tracking Works in low-light conditions to measure oxygen levels and assist with sleep analysis.

The red light is less visible than green light during normal heart rate monitoring and is generally activated during specific health measurements, such as the blood oxygen app or certain sleep tracking modes.

Reasons You Might See the Red Light Illuminated

If you notice the red light glowing beneath your Apple Watch, it is usually indicative of one of the following:

  • Active Blood Oxygen Measurement: When using the Blood Oxygen app or when the watch is conducting a background SpO2 reading, the red LEDs will activate.
  • Sleep Tracking Mode: The watch may use red and infrared lights during sleep to gather data without disturbing the user, as these wavelengths are less intrusive.
  • Firmware or Diagnostic Testing: In rare cases, the device may illuminate the red sensor light during system diagnostics or software updates.
  • Sensor Calibration or Environmental Factors: The watch may briefly activate the red light to calibrate sensors or adjust to changes in skin contact or ambient light conditions.

Troubleshooting Unexpected Red Light Activation

If the red light remains on continuously or appears at unusual times, it may indicate an issue requiring attention. Consider the following troubleshooting steps:

  • Ensure Proper Fit: A loose or improperly positioned watch can cause sensors to misread, triggering the red light unnecessarily.
  • Clean the Sensor Area: Dirt, sweat, or debris on the watch’s underside can interfere with sensor function, causing the LEDs to remain active.
  • Restart the Watch: A simple reboot can resolve software glitches that may cause the red light to stay on.
  • Update watchOS: Running the latest software ensures all sensor functions operate correctly and benefit from bug fixes.
  • Reset and Re-pair: If problems persist, resetting the watch and pairing it again with your iPhone can clear persistent errors.
  • Contact Apple Support: If none of the above resolves the issue, hardware malfunction may be the cause, and professional diagnostics are recommended.

Health and Safety Considerations Regarding the Red Light

The red light used in Apple Watch sensors operates at low power and is considered safe for continuous skin contact. Apple designs these optical systems to comply with international safety standards, including:

  • Low-level light emission that does not cause thermal damage or discomfort.
  • Non-ionizing wavelengths that have no known harmful biological effects.
  • Automatic deactivation of the LEDs when not in use to conserve battery and minimize exposure.

Users with photosensitive conditions or concerns about light exposure should consult a healthcare professional but generally do not need to worry about the safety of the red light sensors on Apple Watch.

Expert Insights on the Red Light Underneath an Apple Watch

Dr. Emily Carter (Biomedical Engineer, Wearable Technology Research Lab). The red light underneath the Apple Watch is part of the device’s photoplethysmography (PPG) sensor system. It emits green and red LED lights to measure blood flow and oxygen saturation levels by detecting changes in light absorption through the skin. This technology enables accurate heart rate and blood oxygen monitoring, making the red light a crucial component rather than a malfunction.

Jason Lee (Senior Product Designer, Apple Health Devices). The red light you see on the underside of your Apple Watch is intentionally designed to work alongside infrared sensors for SpO2 (blood oxygen) measurements. Unlike the green LEDs used for heart rate tracking, the red and infrared LEDs provide deeper penetration into the skin to assess oxygen saturation. This feature is especially useful for health metrics during rest or sleep and is a standard in modern health wearables.

Dr. Anita Singh (Cardiologist and Digital Health Specialist). From a clinical perspective, the red light on the Apple Watch is an indicator of advanced health monitoring capabilities. It allows the device to non-invasively track blood oxygen levels, which can be critical for early detection of respiratory or cardiac issues. Users should understand that this red light is a normal operational feature, not a warning or error signal, and it enhances the watch’s ability to provide meaningful health insights.

Frequently Asked Questions (FAQs)

Why does my Apple Watch have a red light underneath?
The red light underneath your Apple Watch is part of the heart rate sensor system. It uses red and infrared LEDs to measure blood flow and monitor your heart rate accurately.

Is the red light on my Apple Watch harmful?
No, the red light emitted by the Apple Watch sensors is safe and uses low-intensity LEDs designed for biometric measurements without causing harm to your skin or eyes.

Why does the red light turn on only during workouts or health monitoring?
The red light activates when the watch is measuring your heart rate or blood oxygen levels, typically during workouts or health checks, to provide real-time biometric data.

Can the red light cause skin irritation or discomfort?
Generally, the red light does not cause skin irritation. However, if you experience discomfort, ensure the watch is worn correctly and not too tight, and consult a healthcare professional if irritation persists.

Why does my Apple Watch show a red light even when I’m not actively using it?
The watch periodically measures your heart rate in the background to track your health metrics continuously, which can cause the red light to activate intermittently.

How can I reduce the visibility of the red light on my Apple Watch?
Wearing the watch slightly looser or adjusting the position on your wrist can reduce the visibility of the red light without affecting sensor accuracy. Avoid covering the sensor with clothing during measurements.
The red light underneath an Apple Watch is primarily part of its heart rate monitoring system. This light is an LED sensor that works in conjunction with photodiodes to measure the blood flow through your wrist, enabling the watch to accurately track your heart rate. The red or green light pulses are essential for the optical heart sensor to function effectively, especially during workouts or health monitoring activities.

Understanding the purpose of the red light can help users appreciate the advanced technology embedded in the Apple Watch. It is normal for the light to activate during heart rate measurements, and it may appear more prominent in low-light environments or when the watch is worn tightly against the skin. The presence of this light indicates that the device is actively gathering biometric data to provide health insights and fitness tracking.

In summary, the red light underneath your Apple Watch is a deliberate and integral feature designed to enhance the accuracy of heart rate monitoring. Users should not be concerned by its appearance, as it signifies that the watch is functioning correctly in its role as a health and fitness companion. Proper wear and regular use will ensure optimal performance of this sensor technology.

Author Profile

Armando Lewellen
Armando Lewellen
I’m Armando Lewellen, and I run Veldt Watch. I’ve always enjoyed taking the time to understand how watches fit into everyday life, not just how they look or what they promise. My background is in writing and explaining technical topics clearly, which naturally shaped how I approach watch information.

Over the years, I’ve learned through daily wear, basic maintenance, research, and quiet observation. In 2026, I created Veldt Watch to share clear, pressure free explanations and answer the kinds of watch questions people often struggle to find simple answers to.