Your Oura Ring glows green because it’s actively measuring your heart rate. The ring uses green LED lights as part of a sensor system called photoplethysmography (PPG), which shines light onto your skin and reads the reflections to detect your pulse in real time.
How Green Light Measures Your Heart Rate
The ring’s green LEDs shine light into the blood vessels in your finger. When your heart beats, your blood vessels expand slightly with each pulse, and expanded blood vessels absorb more green light than contracted ones. A tiny photodetector on the ring measures how much light bounces back, and the changing pattern of reflected light maps directly to your heartbeat. Oura’s green LEDs operate at a frequency of 50Hz, taking 50 readings per second to build an accurate picture of your heart rate.
This is the same basic technology used in smartwatches and fitness trackers, but the finger is a particularly good spot for it. Arteries in your fingers sit close to the surface, giving the sensor a strong, clean signal to work with.
Why Green Light Instead of Infrared
Your Oura Ring actually has multiple types of LEDs inside it. Green and infrared LEDs track heart rate and heart rate variability around the clock. Red and infrared LEDs measure blood oxygen levels while you sleep. Each color of light behaves differently in your body, and those differences make each one better suited for specific jobs.
Green light has a shorter wavelength than infrared, which means it doesn’t penetrate as deep into your tissue. That sounds like a disadvantage, but it’s actually useful. Because green light stays shallow, it picks up a cleaner signal from the blood vessels near the surface without interference from deeper tissue layers. Infrared light reaches further into the body, where reflections and scattering from non-pulsing tissue create a more complex, noisier signal. Research comparing the two found that green light PPG produced a signal component ratio nearly twice as strong as infrared, and it correlated more closely with actual heart rhythm readings, especially when skin temperature dropped below 15°C (59°F).
The tradeoff is that green LEDs need more power and produce visible light you can actually see. Infrared is invisible to the human eye, which is why you never notice it working. Green light is the one that catches your attention, especially in a dark room.
When the Green Light Activates
The green LEDs turn on throughout the day to take periodic heart rate readings that feed into your 24-hour heart rate graph. They also activate when you use the Live Heart Rate feature. To trigger a manual reading, you tap the heart icon on the Today screen in the Oura app and hold still for about 10 seconds. The ring needs low motion, balanced skin temperature, and a strong Bluetooth connection to get an accurate reading.
During workouts or any time the ring detects elevated activity, the green LEDs may fire more frequently to capture your heart rate in real time. This is part of how the ring tracks metrics like Restorative Time, which monitors periods when your body shifts into a calmer physiological state during the day.
Why It Sometimes Glows at Night
Some users notice the green glow while they’re in bed, which can be startling in a dark room. The green LEDs are primarily designed for daytime heart rate tracking, while the ring is supposed to rely on infrared (invisible) light during sleep. However, the ring’s sensors sometimes activate the green LEDs during sleep, possibly because the ring’s algorithm thinks you’re awake, or because it’s supplementing its readings for better accuracy. If the green glow is disrupting your sleep, wearing the ring with the sensors facing your palm side rather than the top of your finger can help block the visible light from your line of sight.
The infrared LEDs that handle most nighttime measurements are completely invisible, so the ring is often working even when you don’t see any glow at all. The green light is simply the one sensor output bright enough for your eyes to pick up.