Sleep Technology: Gadgets and Products Designed to Enhance Sleep
|by Asleep
Our sleep contains numerous hidden health data. From the time it takes to fall asleep to the efficiency and quality of sleep, disturbances like tossing and turning, snoring, and other sleep events during the night reveal the intricacies of our health.
Previously, we discussed the most precise method for understanding one's sleep, the Polysomnography (PSG) test. PSG involves attaching numerous sensors to the head and body, and it requires spending a night at a specialized sleep medical facility. However, the results might differ slightly from our usual sleep patterns due to the need for wearing various sensors during sleep. Moreover, to obtain more accurate health data through sleep, it's not enough to rely on just one night's data; observing sleep patterns over multi-night is essential.
So, is there a way to accurately assess the depth and quality of our sleep, or perhaps even find ways to improve our sleep through continuous monitoring?
Until now, the most precise way to understand our sleep has been the Polysomnography (PSG) test conducted in specialized sleep medical facilities. During PSG, sensors are attached to various parts of the face and body, including the head, scalp, fingers, and nose. It measures brainwaves, blood oxygen levels, and respiration while monitoring factors like electromyography (EMG), electroencephalography (EEG), electrooculography (EOG), and more. This allows the identification of sleep stages and conditions such as snoring, sleep apnea, and restlessness. However, PSG requires a hospital visit, incurs significant costs, and can only be interpreted by experts, making it impractical for regular and repeated use.
PSG test results that are nearly impossible to interpret without expert analysis
The sleep issues we experience in our daily lives can often be improved not only through surgical treatments or the use of continuous positive airway pressure (CPAP) devices but also through ongoing monitoring of sleep patterns. For instance, when sleep problems manifest as symptoms without underlying medical conditions, early evaluation and observation, along with appropriate interventions, can prevent the condition from worsening.
Advancements in technology now offer us alternatives to the precise PSG test—tools that allow us to monitor sleep patterns over multiple nights without the need for a hospital visit. These tools are commonly referred to as Consumer Sleep Trackers (CST). CSTs are easily accessible to anyone without a prescription and enable simple and convenient nightly monitoring of sleep stages.
Sleep trackers use different biosignals to measure sleep stages. They can be broadly classified into four categories: those tracking body movements during sleep through actigraphy, those tracking heart rate variability (HRV) by measuring slight variations in the time between heartbeats, those tracking respiration, and those tracking sleep through sound.
1. Actigraphy
Actigraphy involves tracking body movements during sleep. This is typically done with wrist-worn devices like actigraphy watches, which use built-in accelerometers to monitor movement. While this method is convenient for providing simple information about whether someone is asleep or awake, it may have limitations. For instance, it might incorrectly measure the time spent lying still in bed, even if the person hasn't fallen asleep yet.
2. Heart Rate Variability (HRV)
Many popular sleep trackers, such as watch or ring types, use this method to measure sleep stages. Because the autonomic nervous system controls heart rate, tracking HRV can provide insights into the autonomic nervous system's state, which, in turn, allows tracking of sleep stages. A high HRV indicates an active nervous system, closer to REM sleep or wakefulness. A low HRV suggests a stable nervous system, associated with light or deep sleep. Machine learning and AI algorithms are often used to accurately interpret HRV patterns.
Unlike the past, when electrodes were required for HRV tracking, modern devices use light-based methods (photoplethysmography or PPG). Have you ever noticed a green light coming from devices like Apple Watch or smart rings? These devices use that light to monitor heartbeats and measure sleep stages.
Sleep trackers measuring sleep stages through PPG
PPG accuracy varies depending on where the sleep tracker is worn. Wearing it on the finger or wrist shows relatively accurate results.
3. Respiration
Similar to heart rate, respiration during sleep provides valuable information about the autonomic nervous system's status. While we can consciously control our breathing while awake, the autonomic nervous system takes over during sleep. Monitoring the regularity of respiration patterns can reveal signals about the autonomic nervous system's activity. For example, if breathing is regular, it indicates deeper sleep stages, while irregular breathing patterns may suggest light sleep or REM sleep.
One common method for monitoring respiration is by emitting radar signals towards the chest and analyzing the reflected signals. This method allows for sleep tracking without wearing any devices, making it advantageous. However, challenges remain, such as improving the size of radar devices and their placement in bedroom environments, despite years of research. Major tech companies like Google Nest and Amazon Halo are leading the way in utilizing radar technology for sleep trackers. In addition to radar, there are methods involving devices placed under the mattress, which monitor respiration through pressure.
4. Sound
The three methods mentioned above can capture various physiological signals such as brainwaves, electromyography, and heart rate variability. These signals can also be monitored through the sounds generated during sleep, including respiratory sounds and various other noises that occur during sleep. For example, as one enters deeper sleep, muscle tension decreases, and airways narrow, resulting in changes in respiratory sounds. Monitoring the regularity of respiratory patterns can provide insights into the autonomic nervous system.
Monitoring sleep stages through the regularity of respiratory patterns
Furthermore, these methods can analyze external sounds, not just biological signals. Sometimes, sounds generated in the vicinity of a sleeping individual can indicate whether they are asleep or awake. The significant advantage of sound-based sleep stage monitoring is that it can be conveniently utilized in everyday life by downloading an app on smartphones, which typically come equipped with high-quality built-in microphones.
참고자료
1. Hong J, Tran HH, Jung J, Jang H, Lee D, Yoon IY, Hong JK, Kim JW. End-to-End Sleep Staging Using Nocturnal Sounds from Microphone Chips for Mobile Devices. Nat Sci Sleep. 2022;14:1187-1201 https://doi.org/10.2147/NSS.S361270
2. Hartmann V, Liu H, Chen F, Qiu Q, Hughes S and Zheng D (2019) Quantitative Comparison of Photoplethysmographic Waveform Characteristics: Effect of Measurement Site. Front. Physiol. 10:198. doi: 10.3389/fphys.2019.00198
3. The Science of Sleep, Heather Darwall-Smith, Sigma Books