Connected rings are no longer just simple electronic gadgets. One of the leaders in this field, Oura, is aiming for a valuation of 15.62 billion dollars with its initial public offering. A connected ring is a ring equipped with sensors that can collect various physiological data. Depending on the model, it can track heart rate, its variability (HRV), skin temperature, physical activity, or even sleep. Most manufacturers recommend wearing the connected ring on the index or middle finger, although some models can be worn on the ring finger. The main thing is to choose a finger where the ring will be well adjusted, not too tight, to maintain regular contact with the skin. The main interest of a connected ring is the continuous monitoring of several physiological parameters, such as sleep, heart rate, and physical activity. These data can help users better understand their habits and, in some cases, modify certain behaviors. Sleep is probably the area where connected rings have been most studied. A systematic review and meta-analysis published in November 2025 compared the data from the Oura Ring with those obtained by reference methods, including polysomnography. The researchers did not observe a significant difference regarding several parameters, such as total sleep duration or its effectiveness. However, Professor Pierre Escourrou notes that validations have often been conducted on specific populations, not necessarily on the general public. Therefore, the results apply to certain types of subjects, and there is a tendency to generalize the performance of these tools to the entire population, even though they have not been validated in that context. Worn every night, the connected ring allows the collection of various data on sleep: bedtime and wake-up times, duration, sleep efficiency, and time spent in bed. This enables users to better understand their habits and identify possible irregularities. The connected ring is, above all, a tool for awareness. It does not allow for improving sleep on its own or treating a sleep disorder, but it can help users understand their habits and track their evolution over time. Depending on the models, connected rings are able to count steps, measure the time spent moving, detect activity, estimate its intensity, and track heart rate. These data allow users to be aware of their level of activity (or inactivity) and can encourage them to move more. Some connected rings also rely on resting heart rate, HRV, body temperature, and sleep data to assess the body's recovery. This allows for better adaptation of physical activity. The collected data also allow the identification of periods where the body shows signs of stress and thus better understand the factors that may influence it. However, note that the ring does not directly measure psychological or emotional stress. It only detects physiological variations that can also be related to other factors, such as intense physical effort, lack of sleep, or a pathological state (cold, etc.). By tracking changes in skin temperature during the night, some connected rings can help estimate the phases of the menstrual cycle, or even predict the date of the next period. A study published in 2025 in the Journal of Medical Internet Research, involving 1,155 cycles in 964 women, showed that the physiological algorithm of the Oura Ring identified 96.4% of ovulations, with an average error of 1.26 days compared to 3.44 days with a method based solely on the calendar. At first glance, connected rings and connected watches fulfill similar functions: both can track physical activity, heart rate, sleep, or certain physiological parameters. The main difference lies more in their use. "Worn on the finger, the ring is more discreet, less bulky than a watch on the wrist, especially during sleep," notes Professor Pierre Escourrou. Reliability varies depending on the parameters measured. Thus, while some general data on sleep or activity are interesting, other indicators are much more fragile. "The detection of sleep relies most often on the actigraphy signal," explains Professor Pierre Escourrou. However, if you are an insomniac, you are not sleeping but also not moving, the ring will consider that you are sleeping, which biases the results. Some rings can also detect signs suggestive of sleep apnea syndrome, notably through the monitoring of oxygenation during the night. "The sensitivity is good for severe forms but less for moderate forms," specifies the specialist. These data are therefore useful for alerting to a possible respiratory abnormality, but they do not allow for a diagnosis. In case of suspicion, a medical assessment and, if necessary, a sleep study remain essential. Finally, if these devices can inform about the duration and effectiveness of sleep, they are much less reliable when it comes to precisely distinguishing its different stages. "Therefore, it is necessary to avoid taking literally the data indicating, for example, the proportion of slow-deep sleep or paradoxical sleep," concludes the specialist. Prices vary depending on the models, features, and finishes offered. As an example, the Oura Ring 4 is available from 349 euros in France, a price to which a monthly subscription of 5.99 euros or an annual subscription of 69.99 euros must be added to access all its features and analyses.