Key Takeaways
1. PPG uses light to detect subtle blood volume changes and translate them into health-related signals.
2. It can support continuous tracking of metrics like heart rate, HRV, blood oxygen, sleep trends, and recovery signals.
3. Because it is non-invasive, PPG fits naturally into everyday wear and long-term health tracking.
4. Smart rings may be an ideal format for PPG, since the finger often provides strong signal quality.
5. Better fit, less movement, and more stable resting conditions can all improve data quality.
5 Min Read
When health metrics have become something you read through numbers, ranges, and labels that suggest whether your body is doing “better” or “worse,” it is natural to wonder:
Where does this data actually come from?
When I am working, resting, or sleeping, how does this small ring read what is happening inside my body?
In this article, we will talk about PPG, or photoplethysmography, an optical sensing technology used to monitor health-related signals.
A Quick Look at PPG
PPG is already used in a wide range of scenarios. Put simply, it works by using changes in light signals to capture the tiny fluctuations in blood volume that occur with each heartbeat. These fluctuations are then translated into readable physiological information.
Today, PPG is widely used in smart rings, smartwatches, and other wearable devices. It helps users track their health continuously, rather than relying on a single “check-up style” measurement taken at one moment in timeIts core functions usually include:
1. Heart Rate
2. Heart Rate Variability
3. Blood oxygen saturation
4. Sleep pattern trends, usually inferred through multiple signals
5. Recovery and stress signals, usually generated through algorithm-based composite metrics
Many people assume that PPG is “just for measuring heartbeats” when they first hear about it.
But in reality, it is more like reading the body’s dynamic curve. Whether your body feels steady, tense, or well-recovered, traces of those states may appear in these subtle fluctuations.
This is where the value of PPG lies. It does not just give you a number. It gives you a trajectory of how your body changes over time.
Why Use PPG?
First, its accuracy is worth trusting in certain use cases.
A study by Luo et al. (2025), published in Annals of Medicine and Surgery, compared the performance of ECG-based smartwatches and PPG-based smartwatches in screening for Atrial Fibrillation (AF).
The results showed that PPG watches achieved a pooled sensitivity of 97.4% and a specificity of 96.6%. Compared with ECG watches, which showed a pooled sensitivity of 83% and a specificity of 88.4%, the advantage was clear for AF detection.
Second, its non-invasive nature brings many practical benefits.
Non-invasive means that once you put it on, it starts working with almost no disruption to your daily life. It does not require blood draws, adhesive patches, or the kind of fixed setup often associated with medical devices.
Because PPG is non-invasive, continuous tracking becomes possible. Health status is not determined by a single moment in time. It is built from patterns that unfold continuously.
Whether you slept well last night, whether you have been feeling more tired recently, or whether your recovery is keeping up often depends on trends, not one isolated reading.
Third, PPG data is highly expandable.
This means that one PPG signal can be used, through later algorithmic processing, to generate multiple health-related metrics.
A systematic review by Almarshad et al. (2022), published in Healthcare, summarized the measurement applications of PPG. It showed that PPG can be used not only for heart rate, but also as a signal foundation for metrics such as HRV, blood oxygen, respiratory rate, and autonomic nervous system-related indicators.
Why Smart Rings Can Be a Strong Use Case for PPG?
The quality of a PPG signal, especially its waveform, can be affected by the contact pressure between the skin and the sensor.
Fluctuations in signal quality may reduce the reliability of more complex monitoring tasks, such as blood pressure and HRV measurement, or even make them difficult to complete (Ho et al., 2025).
However, current research suggests that fingers may be one of the more suitable body locations for PPG.
For example, a study published in Frontiers in Physiology by Hartmann et al. (2019) continuously recorded one-minute PPG signals from six different body locations: the finger, wrist under, wrist upper, arm, earlobe, and forehead. Measurements were taken during both normal breathing and deep breathing.
The results showed that, under both normal and deep breathing conditions, the finger produced the highest level of analyzable PPG waveforms.
This is mainly because the finger has richer blood supply and its bone structure makes it easier for the sensor to stay closely attached.
From a practical wearing experience, rings also have another advantage: they are light, thin, and low-profile. That gives them a “barely there” wearing experience.
And when a device feels almost unnoticeable, it becomes much easier to wear all day. This allows PPG’s strength in continuous monitoring to be fully used, making the collected data and trend interpretation more complete.
You also do not need to deliberately take the ring off before sleep, or tolerate the noticeable presence of a bulkier device while trying to rest.
How to Improve the Accuracy of PPG Monitoring
To get more accurate signals, the first step is simple: wear the ring properly.
Fit matters. The ring should sit snugly, but not feel tight or restrictive.
If it is too loose, light signals can leak. If it is too tight, comfort decreases, and local blood flow may also be affected.
Second, try to reduce motion artifacts caused by intense movement.
PPG is sensitive to movement. Once you start moving, noise can easily mix into the signal. The more intense the movement, the harder the algorithm has to work to separate true physiological signals from interference.
That is why the most stable, high-quality data often appears during sleep, at rest, or during low-intensity daily activities.
Finally, physiological and environmental factors can also affect PPG signals.
Body temperature can influence blood perfusion. Individual differences can also change signal performance. For example, skin tone and peripheral perfusion levels may cause the same device to perform slightly differently from one person to another.
This further supports an important point: comparing your data against your own long-term trends is usually far more valuable than comparing it with someone else’s numbers.
So, if you want to improve accuracy as much as possible, the goal is to create a better environment for the signal to be read:
Wear it securely. Keep movement low. Let the ring capture your body during its quietest moments.
In many cases, the body’s most honest answers appear when you are at your most relaxed.
PPG matters not only because of what it can measure, but because it makes the question of “what is happening inside the body” continuously visible.
While you are asleep, calm, or recovering, it quietly records the changes that the naked eye cannot see, one subtle signal at a time.
Reference list
Almarshad, M. A., Islam, M. S., Al-Ahmadi, S., & BaHammam, A. S. (2022). Diagnostic features and potential applications of PPG signal in healthcare: A systematic review. Healthcare, 10(3), 547. https://doi.org/10.3390/healthcare10030547
Hartmann, V., Liu, H., Chen, F., Qiu, Q., Hughes, S., & Zheng, D. (2019). Quantitative comparison of photoplethysmographic waveform characteristics: Effect of measurement site. Frontiers in Physiology, 10, Article 198. https://doi.org/10.3389/fphys.2019.00198
Ho, M. Y., Pham, H. M., Saeed, A., & Ma, D. (2025). WF-PPG: A wrist-finger dual-channel dataset for studying the impact of contact pressure on PPG morphology. Scientific Data, 12, 200. https://doi.org/10.1038/s41597-025-04453-7
Luo, D. Y., Zhang, Z. W., Sibomana, O., & Izere, S. (2025). Comparison of diagnostic accuracy of electrocardiogram-based versus photoplethysmography-based smartwatches for atrial fibrillation detection: A systematic review and meta-analysis. Annals of Medicine and Surgery, 87(4), 2307–2323. https://doi.org/10.1097/MS9.0000000000003155
