When a group of California researchers started a COVID-19 study in 2020 using fitness tracking rings, they didn’t know they would make a leap forward for an entirely different condition. The researchers set out to learn if off-the-shelf fitness and health trackers worn on the body could be used to detect the early symptoms of COVID-19, according to the study. All participants, 20,880, bought and wore an Oura Ring, a ring that senses such data as heart rate, body temperature, oxygen levels and other fitness indicators, the researchers said. The study found that participants with higher body temperatures also reported higher rates of depressive symptoms and feelings of depression using temperatures taken while users were awake. While it’s not clear if poor body temperature regulation is a symptom of depression or vice versa, the researchers said people with depression overall had a change to their natural immune-based feedback system.