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• A lawsuit targets Oura's allegedly inaccurate sleep tracking features. 
• The plaintiff alleges that Oura's sleep tracking has "a coin flip's chance of being correct."
• Oura has defended its technology and methodology for its smart ring. 

Can a smart ring that measures biometric signals such as heart rate, body temperature, and breathing rate use these data points to accurately determine sleep stages? That question is at the heart of a recent lawsuit in which Oura finds itself entangled. 

Also: I wore the Oura Ring 5 for a month, and two big upgrades make it so much better than the 4

A class action lawsuit filed against Oura alleges that the wearables company used unlawful marketing tactics to mislead consumers about the accuracy of its smart ring's sleep tracking. The Oura Ring, the plaintiff claims, lacks the required sensors and signals, namely, brain wave, eye movement, cardiac activity, muscle movement, and a finger PPG, to accurately measure the neurological activity that defines sleep stages. 

"Oura's AI models are doing guesswork as to what may be happening, based on improper inputs, and thus taking advantage of desperate consumers who struggle with their sleep or are concerned for their health by selling faulty AI-based inference as reliable science," the complaint states. 

The lawsuit primarily targets Oura, but its implications could throw a wrench in other sleep-tracking wearables that use similar sensors to detect sleep stages. Can these signals taken from the wrist or finger really translate into accurate diagnostic data points? 

Madison Surber, the subject of the complaint, bought an Oura Ring last year for $503, assuming it accurately tracked sleep. She claims that she paid "an unwarranted premium" for a device that didn't live up to its marketing promises. The claim cites a Nature study that found the smart ring achieved only "limited accuracy" in sleep staging, with a 53% success rate in correctly assessing each sleep stage. 

"Sleep happens in the brain, not on one's finger," the complaint states. That sentence alone, if proven true in court, could shake up the billion-dollar health wearables industry, whose value proposition hinges on this very idea. Wearable devices like smart rings and smartwatches record biometric signals from the finger or wrist -- such as heart rate, heart rate variability, body temperature, blood oxygen saturation, and respiration rate -- to determine activity, sleep, and even stress. The market's success and growth are predicated on this biometric translation. 

Also: I tracked 3,000 steps on my Apple Watch, Google Pixel, and Oura Ring - this one was most accurate

"We stand behind our science, research, and accuracy claims," Oura told ZDNET. "There is well-documented, peer-reviewed independent scientific evidence that sleep stages are associated with distinct, measurable, reproducible changes in physiology, which is why these signals are reliably used to classify sleep stages."