How Smartwatch Sleep Monitoring Works : Our smartwatches know more and more about us, to such an extent that it is surprising how much they can obtain simply with slight contact with our bracelet. We have all wondered at some point: how does a smartwatch know how well you sleep or are stressed?  If you have also asked yourself that question, we will try to clarify how your smart watch can make more or less precise measurements of your health from its tiny sensors.

Many Measurements, But Not So Many Sensors

In the old days, it was the ultimate when a smartwatch could measure your pulse manually. Later came the continuous pulse record and an explosion of measurements ranging from the quality of sleep to stress, the level of oxygen in the blood or the rate of breathing. There is even a smartwatch capable of measuring your blood pressure, and rumours tell us that the Apple Watch could even show the level of glucose in the blood.

Best value for money smartwatch: recommendations to make the right purchase and eight outstanding smartwatches

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Best value for money smartwatch: recommendations to make the right purchase and eight outstanding smartwatches

Smartwatches today measure everything, yet the sensors are more or less the same. Inside, accelerometers and gyroscopes capture even the most subtle movements. Then, aiming at the wrist, an optical sensor to “look” carefully at your skin and capture the changes of each pulsation or, spinning finer, the microscopic changes in infrared light as it penetrates the blood vessels and returns to the sensor, being able to calculate the blood oxygen level.

Huawei has the TruSeen sensor, Amazfit calls it the BioTracker PPG, and Samsung calls it the Samsung BioActive sensor. None of them goes public with their work or their exact characteristics. Still, they will make up of photodiodes and LEDs to illuminate and look at your skin. Measurements of sleep quality and stress will base on them.

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It is how smartwatch sleep monitoring works

There are several ways to analyze a person’s sleep using technology. For example, some apps use the microphone to detect snoring, but that’s not common on smartwatches. Instead, smartwatches rely on movements detected by their accelerometer, gyroscope, and heart rate measurements.

Amazfit explains it plainly on its website: when the heart rate is low and there is hardly any movement, it will record as deep sleep. It will consider light sleep when there is a lot of activity and the heart rate is high. As sleep measurements spread across millions of devices, Big Data – one of Huami’s businesses – will use  to fine-tune the algorithms that generate results from raw sensors.

The usual thing today is that smartwatches can also estimate when you are in the REM phase. In this case, the pulse and motion sensor data will add to the respiration rate to make the estimate. And how does the smartwatch know your breathing rate? The answer is, again, the accelerometer.

As Huawei explains, the system is far from perfect: your watch won’t know you’ve gotten up if you don’t move. In addition, smartwatches will be aware of how long you sleep in specific time slots (Huawei, from 8:00 p.m.; Amazfit, from 6:00 p.m.), so those who sleep in unusual time slots may have somewhat irregular measurements.

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How Smartwatch Sleep Monitoring Works – Stressed

All the latest smartwatches include stress measurement, which is even more enigmatic than sleep analysis. So how does your smartwatch know that you’re stresse when maybe you haven’t even noticed? On this occasion, the accelerometers can rest since the cardiac sensor is in charge, using the old technique of measuring the alterations in the duration between pulsations, also known as HRV (Heart rate variability).

stress

Harvard Medical School explains it this way:

“HRV offers a non-invasive way of pinpointing imbalances in the autonomic nervous system. For example, based on data collected from many people, the variation between beats tends to be less if the system is in a fight or flight mode. In contrast, the variation can be greater if the nervous system  relaxing.”

Again, the system is far from infallible.

Huawei includes the warning that the consumption of caffeine, nicotine, alcohol or drugs can alter the results, which also will not be very reliable in people with heart disease, or asthma, who are doing sports or do not wear the watch correctly.