Smart rings can be worn more comfortably and discreetly than the best fitness trackers, and when I tried the Oura Ring I was impressed by the higher level of accuracy it offered for tracking sleep compared to sports watches.
I was hoping that the Circular Smart Ring Slim could be just as effective and rival the Oura, but after a month of testing, the Circular Slim failed to impress me with its design and his tracking accuracy is lacking.
Circular Smart Ring Slim: Price and availability
The Circular Ring Slim was released in November 2023 and costs £225 in the UK and €259 in Europe. This is cheaper than the Circular Pro 1 (€364), which is larger and comes in one of four finishes versus the Slim’s. The ring ships worldwide, so it can be purchased in the US.
The Oura Ring 3 costs $299/£229, although you need to have a subscription to unlock many of its best features, which you don’t with the Circular rings.
At the time of writing the Circular Ring Slim was sold outbut you can sign up for notifications about when it’s next available.
How I tried this smart ring
I’ve been wearing the Circular Ring Slim day and night for a month, only taking it off to charge — though that was pretty often. During testing I also wore the Oura Ring 3 to compare readings, as well as the Garmin Epix Pro and the Apple Watch Ultra 2. Apart from the two previous versions of the Oura Ring, this is the only smart ring I’ve tested.
Plan
Slim in the ring name is justified. Where the Oura ring is 2.55mm thick, the Slim is just 2.2mm. It’s also lightweight, at 2g, and comfortable to wear day and night. It is 8.8mm wide and has a small white logo on the top that points upwards that is meant to be worn. To me, this is not an attractive or subtle design feature, although the biggest design problem is the materials used. The Circular Slim is made from a mix of aluminum and carbon fiber, rather than a harder material like titanium, and the result is that it scratches easily and feels plasticky compared to the titanium Oura ring.
The ring broke within days of her wearing it, with silver scratches showing through the black exterior and further reducing the appeal of its design. The ring isn’t waterproof, it’s just water resistant enough to withstand a splash when you wash your hands or take a shower. For now, it’s also only available in one color: matte black.
As with other smart rings, a sizing kit is sent to you when you order to ensure you get the right fit. The ring needs to be tight for the sensors to get an accurate reading, and it’s wise to wear the size kit rings 24 hours a day to see how they feel when your fingers inflate. Slim is packed with sensors to track your sleep and daily activity. It has infrared, red and green LED sensors, a surface temperature sensor and a three-axis accelerometer. There is a tactile button where the logo appears at the top of the ring that you can use to turn off features like the vibrating alarm powered by the motor in the ring.
In the box is a charger that plugs into a USB-C port where the ring is located. The charger is neat and has a key ring on it, but the way the ring has to sit means it’s difficult to use the charger when you’re plugging it into a computer at a desk.
Sleep and activity tracking
Circular Slim tracks a lot of data for you. Built-in sensors that monitor heart rate, heart rate variability (HRV), sleep stages, blood oxygen saturation, stress levels and more. There’s a 14-day calibration period to set your baselines, so it’s only after two weeks that you’ll start getting useful information about your body’s condition.
Headline stats are provided in the app via “Circles”, which give a sleep score, an energy score (similar to the readiness score you get on other devices) and stress levels. You can also take a live measurement with the ring to show your heart rate, HRV and blood oxygen saturation.
You can then click on Cycles for detailed information about your sleep and energy levels. As with other smart rings, I found the sleep analysis the most interesting aspect of the tracker, with information on how long it took me to fall asleep, my total sleep debt and how restless I was during the night, along with more common features like stages of sleep. Slim also tracks HRV and temperature and plots them against your baseline.
Throughout the day, the ring tracks steps, calories burned and heart rate, awarding “cardio points” for when you’re active as well as tracking stress levels. I found that the step tracker generally showed lower totals than the watches I was wearing that day, but not by much. The ring can’t track workouts, nor can it pull workout data from Apple Health or Google Fit to populate that data into the app, unlike the Oura Ring.
Unfortunately, the accuracy problems I saw with overnight ring tracking were extensive. My temperature fluctuated wildly during testing, including one night where it read 7.3°C (about 12°F) below normal, while other trackers and the Oura ring showed normal readings.
I only get about a minute of deep sleep during the month according to the ring, again when other trackers have noticed I usually get 40-50 minutes a night. My sleep scores also sometimes failed to live up to reality: I was getting 90+ scores for a week, despite being regularly woken up by a baby and staying up late drinking alcohol one night. My resting heart rate data is also higher than with other devices and some nights I get no sleep data from the Slim stages or no sleep at all.
Some data was accurate or at least consistent with other devices, such as the times I slept and woke up, although the Circular Slim missed the times I woke up at night occasionally, and the HRV readings were similar to those of the Oura Ring and Garmin Watch.
The application
The Circular app is busy, which isn’t surprising given the amount of data the ring collects, but it’s a little cluttered, and once you look past the Circles at the top of the app, the data is overwhelming. The Oura app is cleaner and more pleasant to use, though at least the Circular app doesn’t require a subscription to use.
However, it takes time for the ring to sync with the app and sometimes I couldn’t find time to wait for the data to show up in the morning. The ring stores 20 days of data so you can sync elsewhere, but this slow sync is again worse than the experience with the Oura Ring, which flashes sleep and readiness scores shortly after opening the app on morning.
There were also bugs with the Circular app, like frequent notifications that I was offline (when I wasn’t) and that I couldn’t rename the ring (after it told me I could rename the ring when I first set it up).
A new feature of the Circular Slim is the AI assistant Kira+, which offers daily recommendations based on the data the ring tracks. Unfortunately, this was not as clever as I had originally hoped. Kira+’s suggestions were a long stream of text telling me, once again, the data the ring has tracked, as well as basic tips like doing breathing exercises and keeping your bedroom cool and comfortable at night.
You can set an alarm in the app, including a smart wake-up function that wakes you up at a time based on your sleep patterns. The ring vibrates to do this and you turn off the vibration by tapping the logo. It’s a discreet way to wake up when others are in bed with you.
Battery life
The Slim is reported to last up to five days on a charge, but it’s in Eco mode, which lowers the heart rate and doesn’t track your sleep stages. In Performance mode the ring lasted two days (at most), which is less than the four days I usually get from the Oura ring.
Is the thin circular smart ring worth it?
I would avoid Circular Slim. To me the design is not particularly nice and scratches easily and I have reservations about the accuracy of its tracking. Battery life is short and the Oura Ring is simply a better all-rounder. I have yet to try other rings like the Ultrahuman Ring or the RingConn Smart Ring, which might be better alternatives to the Oura than the Circular Slim.
