OURA Ring 4 Smart Ring - Stealth - Size 9

Table 1What it measures, and how

Sensor list and specifications from Oura's Ring 4 store page, the Oura support page on body temperature, and the Amazon listing, all read on Sep 30, 2026. Everything the app shows is an estimate calculated from light, motion and skin-temperature signals.

Strip 1Accuracy strip

Each cell is one published comparison against a reference method. Only one cell tested Ring 4 itself; the rest used earlier Oura rings, and results from one group of volunteers may not transfer to you.

• Independent peer-reviewed study
• Manufacturer or regulatory filing data
• No published data found

• Sleep vs. wake, epoch by epochAccuracy 91.7–91.8%vs. ambulatory polysomnography, up to 3 nights, n = 96; tested Gen3; funded by Oura[1]
• Total sleep time, older adultsBias −75.5 minvs. home polysomnography, 1 night, n = 15 analyzed (ages 56–80); young adults −15.5 min, n = 12; tested Gen3; NIH-funded[2]
• Nighttime heart rate, 5-min windowsBias −0.44 bpmvs. chest-electrode reference at home, n = 35; data from 2019 (pre-Gen3 ring)[4]
• Heart rate, rest to walkingMAPE 15.0% (median)vs. chest-strap electrode reference, climate chambers, 10 sessions; tested Gen3[5]
• Nightly HRV (RMSSD), at homeCCC 0.99; MAPE 5.96%vs. Polar H10 electrode chest strap, 13 adults, 139 Ring 4 nights; tested Oura Gen 4; funded by the US Air Force Research Laboratory[11]
• Ring 4, sleep stagesNo published data foundPubMed search for Ring 4 / fourth-generation Oura sleep-staging studies, Sep 30, 2026

Only the nightly HRV cell tested Ring 4 (the paper calls it “Oura Generation 4”). Three cells used Gen3 and one an earlier ring. Oura says Ring 4 uses a new “Smart Sensing” technology, so the older results describe the product line, not this exact model.

NotesReading the evidence

Why the finger. A ring's optical sensor sits against the palm side of the finger, where the light path is short and the ring moves less than a watch during sleep. The same site has costs. The authors of the independent heart-rate study [5] point out that finger blood flow is sensitive to temperature and that a ring has a smaller optical contact area. In their chamber protocol the Gen3 ring had a median absolute error of 11.0 bpm and a median MAPE of 15.0% across rest, a mental task and walking, one of the weaker results among the ten devices tested. Only ten sessions were run with the ring, so the estimate is imprecise.

Overnight heart rate is a different job. In a home study run by university researchers in Finland and California, 35 adults wore an Oura ring and a chest-electrode recorder overnight [4]. Heart rate from the ring tracked the reference closely: mean bias −0.44 bpm and a Pearson correlation above 0.99 in 5-minute windows. RMSSD, the HRV value the app shows, also correlated well, but other HRV measures computed from 5-minute windows had wide error. The data were collected in 2019, so the ring was an earlier model; the paper does not name the generation. The authors thank Oura for data access; the funding was a US National Science Foundation grant. The one study we found that tested Ring 4 itself, funded by the US Air Force Research Laboratory, compared nightly values with a Polar H10 electrode chest strap in a group of 13 adults sleeping at home [11]. Over 138 to 139 Ring 4 nights, its nightly resting heart rate reached a concordance of 0.98 (MAPE 1.94%) and its nightly RMSSD 0.99 (MAPE 5.96%), the closest agreement among the wearables tested. These are whole-night values from a small group.

Sleep staging, funded and independent. Polysomnography (brain, eye and muscle signals scored by a technician) is the reference for sleep stages. The largest Gen3 study, from a University of Tokyo group with financial support from Oura, recorded 96 adults with ambulatory polysomnography for up to three nights [1]. It found no significant difference from polysomnography for total sleep time, light sleep or deep sleep. Epoch-level accuracy for individual stages ranged from 75.5% (light sleep) to 90.6% (REM). A single-night lab study of 35 adults at a Boston hospital, also funded by Oura, reported stage sensitivities of 76.0–79.5% for the Gen3 ring [3]. An independent study funded by the US National Institute on Aging tested Gen3 in the homes of young and older adults [2]. In young adults the ring's total sleep time was close to the reference. In older adults it underestimated total sleep time by 75.5 minutes on average and overestimated deep sleep; the authors advise caution when reading consumer sleep data in older age groups.

Temperature is a trend, not a thermometer. Oura's support page says the ring measures skin temperature at the finger overnight and shows the difference from your personal baseline, which forms over the first couple of weeks. It says this is not the same as core body temperature [8]. Read the number as a relative signal. It is not a temperature reading you can compare with a thermometer. The ring and app have other features outside this entry's scope.

ProcedureGetting cleaner readings

Setup steps from Oura's store page and the Amazon listing. They help the sensors keep contact; they do not guarantee accuracy.

1. Size before you buy

   Oura sizes differ from standard ring sizes, and each size is a separate Amazon listing. Oura says to wear a sizing-kit ring for at least 24 hours, including overnight, and to pick the smaller size if you fall between two [6].

2. Keep the sensors against the palm side

   The sensors are on the inside of the band. A snug fit that does not spin keeps them on the skin; a loose ring lets light leak in and adds motion noise.

3. Give the baseline time

   Temperature and other values are shown against your own baseline. Oura says the temperature baseline forms over the first couple of weeks and may need to re-form on a new or replacement ring [8].

4. Wear it the same way each night

   Same finger, same hand. Comparisons between nights are only as consistent as the placement.

LogPrice and where it is sold

On Sep 30, 2026 the Amazon listing "OURA Ring 4 Smart Ring - Stealth - Size 9" (ASIN B0D9WVKRKS) showed $349.00, shipped and sold by Amazon.com, in stock, with a free 30-day refund or replacement. Oura's own store showed the same $349 for the Stealth finish that day. The ring alone is not the full cost. After the first free month, the membership is $5.99 a month or $69.99 a year before tax for US members [7]. At the annual rate, two years of use adds $139.98 to the purchase price.

Who should look elsewhere

• Anyone who needs a sleep or heart measurement for a medical decision. A clinician can arrange a sleep study or an appropriate device.
• People who do not want a recurring subscription. A watch or tracker whose sleep and heart-rate data work without a membership may suit them better.
• People who want accurate heart rate during workouts. A chest strap such as the Polar H10 is the usual reference in validation studies.
• Anyone unsure of their size who cannot wait for a sizing kit, since each size is sold as its own listing.

Sources

1. University of Tokyo group, with financial support from Oura Health Oy. Oura Ring Gen3 with its 2.0 sleep-staging algorithm compared with multi-night ambulatory polysomnography in 96 healthy Japanese adults aged 20–70. Sleep Medicine, 2024. PMID 38382312. link
2. University of Massachusetts Amherst. Oura Ring (3rd generation) and three other consumer sleep trackers compared with one night of home polysomnography in young (19–24) and older (56–80) adults; NIH-funded, no financial disclosures. Sleep Advances, 2026. PMID 41768372. link
3. Brigham and Women's Hospital. Oura Ring Gen3, Fitbit Sense 2 and Apple Watch Series 8 compared with in-lab polysomnography in 35 adults; funded by Oura Ring Inc. Sensors, 2024. PMID 39460013. link
4. University of California, Irvine and University of Turku. Oura Ring nighttime heart rate and HRV compared with a chest-electrode recorder at home in 35 adults; NSF-funded, no conflicts declared. Journal of Medical Internet Research, 2022. PMID 35040799. link
5. KU Leuven and Thomas More University of Applied Sciences. Heart rate from ten wearables, including Oura Gen3, compared with a chest-strap electrode reference across climate conditions and activities in 45 adults; no conflicts declared. JMIR Formative Research, 2026. PMID 41701929. link
6. Oura. Oura Ring 4 Stealth store page: sensors, battery, water resistance, sizing. Accessed Sep 30, 2026. link
7. Oura. Membership page: pricing by region. Accessed Sep 30, 2026. link
8. Oura Help Center. Body Temperature (last updated April 24, 2026). Accessed Sep 30, 2026. link
9. Oura. Terms and Conditions. Accessed Sep 30, 2026. link
10. U.S. Food and Drug Administration. 510(k) Premarket Notification database; no entry found for Oura as applicant or device name (queried via FDA's openFDA 510(k) data, Sep 30, 2026). link
11. The Ohio State University and the Air Force Research Laboratory. Nightly resting heart rate and HRV from five wearables, including Oura Generation 4 (Ring 4) and Generation 3, compared with a Polar H10 chest-strap reference at home; AFRL-funded, no competing interests declared. Physiological Reports, 2025. PMID 40834291. link

This entry is educational and is not medical advice. Consumer wellness gadgets are not diagnostic devices; readings can be wrong. If a reading worries you, or you have symptoms, talk to a qualified clinician rather than relying on a gadget.