Last updated: 2022/3/25
I bought two smart bands to monitor sleep and heart rate. It’s interesting to know which sensor is usedI bought two smart bands to monitor sleep and heart rate. It’s interesting to know the sensors and medical theories behind them.
After using them for more than 10 months, I realize alarms, timers, sleep, and exercise trackers are the most frequently used features to me. The latter two are good at recording durations but not accurate on sleep stages and heart rate. I also found the gyroscope helps to detect motions more precisely than a single accelerometer.
In conclusion, smart bands are good at time management but not heart rate or sleep stages. The sensors and theories are not as accurate to medical equipment.
Table of Contents
- Quick Guide
- Sleep Tracker
- Heart Rate Tracker
- Exercise Tracking
- For tracking sleep stages, most smart bands using PPG, accelerator, and gyro sensors have less than 60% accuracy compared to a EEG device.
- For monitoring hear rate, ECG is more accurate than PPG.
- While exercising, PPG has medium error rate on heart rate and SpO2.
- SpO2 can be detected by PPG with infrared and red light.
- There are transmissive and reflective PPG. Most smart bands use the later.
- A gyro sensor provides more useful information than an accelerometer while detecting activities.
When my sleep quality is poor, my eyes are swelling due to the increased intraocular pressure (IOP). It makes me feel sickness and headache.
Besides, I also have obstructive sleep apnea (OSA). Sometimes I would wake up suddently and eager for fresh air.
I have read several reports shared by doctors about glaucoma and OSA. Use side sleep to decrease the chance of OSA and improve sleep quality.
To monitor, I bought a realme Band and a Samsung Galaxy Fit 2. With two smart bands, it’s easier to compare and select which one to keep. They both monitor sleep cycles based on actigraphy which is good on sleep time but not as accurate on cycles.
|Feature||realme Band||Galaxy Fit 2|
Both the sleep cycles and sleep heart rate if the “24-hour HR monitoring” is enabled.
Sleep cycles only.
|Turn off wake up screen||Yes.|
Enable realme Link ➞ [realme Band] ➞ [⚙] ➞ [Raise to wake] and setup the schedule. For time outside the in effect period, no wake up.
Disable Galaxy Wearable ➞ [Advanced] ➞ [Wake-up gesture]
|Turn off notification||Yes.|
Disable realme Link ➞ [realme Band] ➞ [⚙] ➞ [Notifications] or enable realme Link ➞ [realme Band] ➞ [⚙] ➞ [Do not disturb] and schedule.
Disable Galaxy Wearable ➞ [Notifications] or enable
Galaxy Wearable ➞ [Advanced] ➞ [Do not disturb] and schedule
It can be enabled by the ⛔ on the band, too.
|Sleep interrupt during testing||Yes.|
Even both wake up screen and notification are disabled, it vibrates and flashing green light for unknown reason. Maybe wake up and long press accidently.
When the “Good night mode” is enabled, the brightness is set to the lowest, wake-up screen and vibration are disabled.
It can be enabled in Galaxy Wearable ➞ [Advanced] ➞ [Good night mode] or 🌙 on the band.
Enable realme Link ➞ [realeme Band] ➞ [⚙] ➞ [Brightness] ➞ [Night Brightness] and schedule.
But it cannot be scheduled, manually enable the [Good night mode] instead.
|Analytical Report||Sum and percentage of sleep stages.||Sum, percentage, and score of sleep stages. Compare to people with similiar ages. More data and terminology explainantion in plain English.|
|Advice||N/A||Only the latest sleep report displays advice from National Sleep Foundation.|
realme doesn’t explain how it gets these numbers and what they mean to me. REM data seems inaccurate. It is “6 minutes” every night during test. realme also detects much longer deep sleep than Samsung.
Your sleep score is based on heart rate and motion data.Samaung Health ➞ [Sleep] ➞ [Sleep details] ➞ [Learn more] ➞ [ℹ]
Samsung provides more insight, advice, and a sleep score for each record. But when there is more than 1 sleep record at night, the sleep scores are confusing. Samsung detects much shorter deep sleep than realme.
Sleep is not uniform. Instead, over the course of the night, your total sleep is made up of several rounds of the sleep cycle, which is composed of four individual stages. In a typical night, a person goes through four to six sleep cycles.
We spend the most time in deep sleep during the first half of the night. During the early sleep cycles, N3 stages commonly last for 20-40 minutes. As you continue sleeping, these stages get shorter, and more time gets spent in REM sleep instead.Sleep Foundation: Stages of Sleep
The total deep sleep (aka N3, Slow-Wave Sleep (SWS), Delta Sleep) is 80 to 240 minutes every night. Samsung indicates the “Typical range” for each stage when the total sleep is more than 4 hours in a sleep record.
Both continue tracking sleep cycles after disabling heart rate monitoring. Samsung continue its stress monitoring which is measured by heart rate changes in a give time period, too. I notice there is flashing green light on Samsung. The PPG is working and doesn’t save the heart rate data.
Because both smart band are not measuring the delta wave directly, it’s better to take these data as a reference.
A Different Approach
The Quantified Scientist uses HypnoDyne’s ZMax, an electroencephalography (EEG) device, in Samsung Galaxy Fit2 Science Test. It measures voltage fluctuations resulting from ionic current within the neurons of the brain. The rythum activities can be detected and classfied into frequency bands. The delta band can be used to identify sleep stages.
Slow-wave sleep (SWS), often referred to as deep sleep, consists of stage three of non-rapid eye movement sleep. Initially, SWS consisted of both Stage 3, which has 20–50 percent delta wave activity, and Stage 4, which has more than 50 percent delta wave activity.Wiki: Slow-wave sleep
Stage N3 is defined by the presence of 20% delta waves in any given 30-second epoch of the EEG during sleep, by the current 2007 AASM guidelines.
He also use a confusion matrix to compare sleeping data from both devices. It helps to know the probability a stage is detected as others.
Heart Rate Tracker
I feel palpitation, perspiring, chest pain, fear, anxious, and panic with some pseudoephedrine in the Second-generation H1-antihistamines Antihistamine. Therefore, when there is a new prescription, I need to monitor heart rate carefully.
I also need to monitor during sleep due to the OSA which cause rapid hearbeat.
Palpitations are perceived abnormalities of the heartbeat characterized by awareness of cardiac muscle contractions in the chest, which is further characterized by the hard, fast and/or irregular beatings of the heart.Wiki: Palpitations
|Feature||realme Band||Galaxy Fit 2|
|All-day Monitoring||Managed in realme Link ➞ [realeme Band] ➞ [⚙] ➞ [24-hour HR monitoring]||Managed in Galaxy Wearable ➞ [Health settings] ➞ [Auto heart rate settings]|
|Time||5, 10, 20, 30 minutes||Continuously, 10, 30 minutes|
|Alert||High: 100, 120, 130, 140, 150 bpm|
Low: 40, 45, 50 bpm
|Only when set to “Continuously” may enable the “High heart rate alert”. |
Above 120 bpm for 10 minutes.
As far as reliability though, we found that it tended to report a higher resting heart rate on average, some times at more than 20bpm compared to another heart rate monitor.Wareable: Realme Band review: not the ultimate budget fitness band
If you care about tracking your heart rate, whether it’s during workouts or just through the day and night, we’d say based on our experience, it’s not quite fit for that. Daily resting heart rate readings compared to the Fitbit Sense and Garmin Fenix 6 was generally around 10bpm higher, which is a huge difference.
When you switch to exercise tracking, we took it out on several runs and average and maximum heart rate readings were way off a Garmin HRM Pro chest strap monitor. As much as 20bpm. Switching to some HIIT workouts on the FIIT home workout app, it was a similar story. It posted lower average heart rate and maximum heart rate readings compared to the Apple Watch Series 6, which houses one of the most reliable wrist-based monitors.TechRadar: Samsung Galaxy Fit 2 review
Omron HEM-7111 Blood Pressure Monitor is a digital device used in medical centres as well as home applications. It works on the principle of Oscillometry with pressure gauge capable of measuring the systolic and diastolic values in 5 seconds. The digital blood pressure monitor has an accurate calibration to level of ± 3mmHg. It can also measure Pulse with a precision accuracy of 5 percent buffer.Amazon: Omron HEM-7111 Blood Pressure Monitor
realme provides similar data to Samsung during test. Both report 2 to 6 bpm higher than Omron HEM-7111 at rest.
When I feel uncomfortable or the numbers is abnormal, I will use Omron HEM-7111 for double check. It’s a consumer grade medical equipment. Omron provides free re-calibration service on certain models due to sensor aging.
A Different Approach
The Quantified Scientist uses Polar H10 Heart Rate Monitor which contains a hospital grade electrocardiogram (ECG/EKG) monitor in his Samsung Galaxy Fit2 Science Test. It measures voltage of the electrical activity of the heart using electrodes placed on the skin.
With ECG, HR can be measured accurately. HR can be measured with PPG, however it is only suitable for average or moving average measurements.NeuroSky: ECG vs PPG for Heart Rate Monitoring: Which is Best?
Pulse meters are not very reliable when used out of doors because of the changes in ambient light. In addition, they are rather sensitive to body movements and are not accurate during intensive exercise. This is because of the movement of the device during exercise and because the blood flows smoothly without distinctive intervals in the thin blood vessels of the earlobe.Polar: Difference Between Heart Rate and Pulse
Compared to PPG, ECG is more accurate at any time.
Exercise tracking is not my primary interest. I only test the “Walking” mode. Samsung is smart to stop counting when I am waiting for a traffic light. It would be more accurate monitoring my time on activities.
Smart bands rely on sensors and algorithm to track and identify activities. There is no information about the sensor model. Wondering if a more expansive smart band / watch uses more accurate sensors.
|Sensor||realme Band||Galaxy Fit 2|
|PPG (photoplethysmography) optical heart rate sensor||Yes||Yes|
|3-axis gyro sensor (gyroscope)||N/A||Yes|
PPG is a technical term for shining light into the skin and measuring the amount of light that is scattered by blood flow. That’s an oversimplification, but PPG sensors are based on the fact that light entering the body will scatter in a predictable manner as the blood flow dynamics change, such as with changes in blood pulse rates (heart rate) or with changes in blood volume (cardiac output).Valencell: Optical Heart Rate Monitoring: What You Need to Know
There are apps use the flash and main camera to do the same trick. Search ‘heart rate monitor’ in Google Play.
A photoplethysmograph (PPG) is a simple medical device for monitoring blood flow and transportation of substances in the blood. It consists of a light source and a photodetector for measuring transmitted and reflected light signals. Clinically, PPGs are used to monitor the pulse rate, oxygen saturation, blood pressure, and blood vessel stiffness.PubMed Central: Current progress of photoplethysmography and SPO2 for health monitoring
Not only heart rate but also Oxygen Saturation can be tracked by PPG. More precisely, it’s the peripheral oxygen saturation (SpO2).
Pulse oximetry uses light to work out oxygen saturation. Light is emitted from light sources which goes across the pulse oximeter probe and reaches the light detector.
If a finger is placed in between the light source and the light detector, the light will now have to pass through the finger to reach the detector. Part of the light will be absorbed by the finger and the part not absorbed reaches the light detector.
The amount of light that is absorbed by the finger depends on many physical properties and these properties are used by the pulse oximeter to calculate the oxygen saturation.how equipment works .com: How pulse oximeters work explained simply
The big different between tracking heart rate and SpO2 is the former needs reflective light while the later needs to pass through to see what has been absorbered.
Since the maximum pulsatile component of reflected light occurs approximately in the range between 510 and 590 nm, the green (565 nm) or yellow (590 nm) light is generally used for reflective PPG sensors. However, the red (680 nm) or near-infrared (810 nm) light is generally used for transmissive PPG devices, with the infrared light having the deepest penetration.Nature Portfolio: The use of photoplethysmography for assessing hypertension
Both realme and Samsung use green light in PPC. There is no red available. Mi Smart Band 6 has a green light for heart rate and red for SpO2.
Red and infrared light can penetrate about 2.5 mm, while green light can penetrate less than 1 mm into tissue. Hence, the detection of blood pressure, atherosclerosis, blood sugar, and other physiological parameters uses the infrared light (deeper light penetration compared with the green light) to obtain PPG signals.Nature Portfolio: The use of photoplethysmography for assessing hypertension
Because green is weaker than red on penetration, it cannot provide enough accuracy with a firmware upgrade.
The study found that there were correlations between the standard OSA severity based on AHI severity classification and different SpO2 and polysomnographic parameters, including the desaturation index (DI), the sum of all desaturations, desaturation below 90%, the average duration of respiratory events, and indices of total arousals and respiratory arousals. All of these parameters correlated directly with OSA severity classification (P < 0.001 for each). However, REM sleep duration and SpO2 nadir were inversely correlated with OSA severity (P < 0.003 and < 0.001, respectively). In addition, only the DI, SpO2 nadir, and respiratory arousal index were predictors of OSA severity, as determined through a multiple logistic regression analysis.PubMed Central: The correlation between oxygen saturation indices and the standard obstructive sleep apnea severity
SpO2 is an important index to sleep quality and OSA. Not all smart bands monitor SpO2 during sleep. Some supports single-time measuring only. I also found two interested threads: Huawei GT2 and Band 4 doesn’t provide SpO2 detect in Japan, Korea, and Taiwan and Huawei Watch GT 2e has SpO2 feature enabled after firmware upgrade. Maybe it’s related to local regulation of medical devices.
Our results show that the forehead produced the most accurate results for HR and SpO2 both at rest and walking, however, it had poor RR results. The finger recorded similar results for HR and SpO2, however, it had more accurate RR results.MDPI: A Comparison of Reflective Photoplethysmography for Detection of Heart Rate, Blood Oxygen Saturation, and Respiration Rate at Various Anatomical Locations
There are transmissive and reflective PPG. The report shows PPG is good detecting SpO2 at rest but the error rate increase while walking.
An accelerometer is a device that measures the vibration, or acceleration of motion of a structure.Omega Engineering: How to Measure Acceleration?
It detects the linear velocity change on static and dynamic acceleration.
There are many different types of gyro sensors. Vibration gyro sensor is used in mobile devices.
It detects the angular velocity. A processor will use it to calculate the angle and force. These data can be used on motion sensing in a smart band.
It’s the key factor why Samsung is more accurate than realme on detecting activities.
I have encountered two issues with realme. I am not sure it’s due to the app or the band.
Fail to link realme Band
realme Link fails to link the band. It also displays SpO2 history with empty data. The app is disappeared in [Settings] ➞ [Apps] ➞ [Apps]. I cannot reinstall from Google Play, either.
It goes back to normal after restarting the phone.
Invisible Alarm in realeme Band
One day the realme band vibrate and pop-up an alarm which I have never created. There is no alarm in realme Link ➞ [realeme Band] ➞ [⚙] ➞ [Alarms].
To remove the invisible alarm, I create a new alarm at the same time. Sync to the smart band. Then delete and sync again. It doesn’t bother me anymore.
Walking Stop Counting while Connecting to Galaxy Wearable
On 2021/5/28, I want to track walking-in-place with both smart bands. They are both connected to the same smartphone via Bluetooth. Galaxy stopped counting after a few seconds and didn’t restart. I have to choose “Other workout” instead of “Walking” to track.
After more investigation, I found “Walking” works correctly if disconnected. If the exercise isn’t tracked properly, try to disconnect before starting.
- Wiki: Intraocular pressure
- Wiki: Obstructive sleep apnea
- Wiki: Glaucoma
- realme Band
- Samsung Galaxy Fit 2
- YouTube: Sleep Expert Answers Questions From Twitter｜GQ Taiwan
- Verywell Health: The 4 Stages of Sleep (NREM and REM Sleep Cycles)
- Wiki: National Sleep Foundation
- Sleep Foundation: Stages of Sleep
- HypnoDyne: ZMax
- Wiki: Eelectroencephalography
- The Quantified Scientist: Samsung Galaxy Fit2 Science Test: Watch Before You Buy! (2021 Review)
- Wiki: Slow-wave sleep
- Wiki: Confusion matrix
- Healthline: How Much Deep, Light, and REM Sleep Do You Need?
- Wiki: Palpitations
- Wiki: Pseudoephedrine
- Wiki: Second-generation H1-antihistamines
- Wiki: Antihistamine
- Wareable: Realme Band review: not the ultimate budget fitness band
- TechRadar: Samsung Galaxy Fit 2 review
- Amazon: Omron HEM-7111 Blood Pressure Monitor
- Polar: Polar H10 Heart Rate Monitor
- Polar: White Paper: Polar H10 Heart Rate Sensor System
- Qardio: How does ECG/EKG work and what is an ECG/EKG monitor?
- Wiki: Electrocardiography
- NeuroSky: ECG vs PPG for Heart Rate Monitoring: Which is Best?
- Polar: Difference Between Heart Rate and Pulse
- GSMArena: Realme Band Review
- Valencell: Optical Heart Rate Monitoring: What You Need to Know
- PubMed Central: Current progress of photoplethysmography and SPO2 for health monitoring
- Wiki: Oxygen saturation
- how equipment works .com: How pulse oximeters work explained simply
- Nature Portfolio: The use of photoplethysmography for assessing hypertension
- PubMed Central: The correlation between oxygen saturation indices and the standard obstructive sleep apnea severity
- MDPI: A Comparison of Reflective Photoplethysmography for Detection of Heart Rate, Blood Oxygen Saturation, and Respiration Rate at Various Anatomical Locations
- Huawei: SPo2 Features Missing In GT2 And Band 4
- Huawei: [HUAWEI Watch GT 2e] SpO2 Monitoring | Measure Blood Oxygen Saturation Levels at your wrist!
- Omega Engineering: How to Measure Acceleration?
- Epson: Gyro sensors – How they work and what’s ahead
- Wiki: Angular velocity
- Wiki: Coriolis force
- The Times of India: 16 sensors that are present inside fitness bands and smartwatches that you need to know
- Style Guide