The principle of smart bracelet measuring sleep quality

In modern society, people are under increasing pressure, and sleep quality is particularly important. Tracking and recording sleep conditions can help us better regulate our sleep. In recent years, the birth of smart bracelets and smart watches has enabled us to always pay attention to our heart rate and sleep quality. So, have you ever thought about how a small smart bracelet or watch can do this?

For sleep quality, smart bracelets or watches are achieved through actigraphy. Actigraphy was first developed by the US military in the 1980s to measure troop performance. Later, researchers found that actigraphy can be used in many fields and gradually introduced it to the market. After more than 30 years of improvement, it has been widely used in many fields such as sleep/wake pattern assessment, daily activity intensity monitoring, drug testing, behavioral genetics, etc. based on its advantages such as portability, non-invasiveness and low price.

We know that during light sleep, the human body will produce some slight movements. The body movements detected by the smart bracelet at this time will mostly judge the wearer as being in a light sleep state; while during deep sleep, the muscles of the person will relax, and the limbs will not produce large movements, or even not move. At this time, the sleep bracelet records less body movements, which is considered deep sleep. Therefore, the monitoring of sleep quality is actually sensing whether our body has relatively large movements during sleep. Sometimes, turning over with a bracelet on is actually a deep sleep state, but because it "moves", the smart bracelet will also identify it as light sleep. At present, most body motion recorders have the ability to record data from three directional axes, and three-axis sensors have become standard configurations. The three-axis sensors can sensitively record slight movements from three directions. Based on the recorded data, the analysis software can calculate and analyze energy consumption and sleep-related parameters, awakening time, number of awakenings, sleep efficiency, etc.

As for heart rate monitoring, there are currently two main methods: one is the photoelectric transmission measurement method, and the other is the electrocardiogram signal measurement method. The so-called photoelectric transmission measurement method is that the regular beating of the heart will cause changes in the light transmittance of the blood. The information of this change is collected and converted into an electrical signal, which corresponds to the heart rate information. It is a technology that measures the pulse in an optical way. Simply put, there is a photoelectric receiver on the back of the sports bracelet that emits green light. After the light beam is emitted, the skin, muscle tissue and blood will absorb a part of it, and the rest will be reflected back to the receiver. Blood tends to reflect red light and absorb green light, so the heart will produce reflected light of different colors when it contracts and relaxes. The bracelet detects these reflected lights and then uses a certain algorithm to convert the heart rate.

Careful friends may find that we often see green LEDs on sports bracelets. Why is that? This is because the absorption rate of green light is the highest in front of red liquid such as blood, and it is more accurate for data judgment. When the user's heart beats, more blood will flow through the user's wrist, and the amount of green light absorbed will also be greater; in the interval between heart beats, the blood flow decreases, resulting in less absorption of green light.

The second method is the ECG signal measurement method, which is similar to the ECG test commonly used in hospitals. The difference is that hospitals need to add electrodes to the chest and limbs and need to measure more data, while wrist watches do not need so much data, only the heart rate is enough. Human tissues and body fluids can conduct electricity. The heart is like a power source. The potential changes of countless myocardial cells can be transmitted and reflected on the body surface. There is a potential difference in different parts of the body surface. Through the collected electrode changes and algorithm processing, the heart rate value can be restored.