Fitbit Charge 2: Sleep Tracker Functionality Explained

The Fitbit Charge 2 is a multisensory device that has shown promise for clinical sleep tracking. It can be used to automatically detect and store sleep data via the Fitbit app, making it a less expensive and intrusive option for patients to collect sleep data. While the Fitbit Charge 2 has been found to accurately estimate the amount of time spent in REM sleep, it has also been found to overestimate the time spent in light sleep and wakefulness after sleep onset. It is also not very accurate in differentiating between restlessness and awake time. To accurately track sleep, the device should be positioned higher on the wrist, and the battery should be charged to at least 40% before going to bed.

Fitbit Charge 2 sleep tracking accuracy

The Fitbit Charge 2 is a multisensor fitness tracker that can be used to monitor sleep quality. It can track sleep stages such as light, deep, and REM sleep, as well as time awake, by using heart rate data. The device also provides guided breathing sessions, which can help users relax and fall asleep.

To track sleep with the Fitbit Charge 2, users must wear the device during sleep. The Fitbit app allows users to manually adjust sleep settings and set bedtime and wake-up reminders. The app also provides a sleep score and a detailed sleep breakdown, including a sleep animal characterisation of long-term sleep behaviours.

The accuracy of the Fitbit Charge 2's sleep tracking has been evaluated in a study of shift workers, including police officers and paramedics. The study compared the Fitbit Charge 2's sleep estimates with polysomnographic (PSG) measures. While the Fitbit Charge 2 provided reasonably accurate mean values of sleep and heart rate estimates, there were some discrepancies in the distribution of sleep episode durations between the Fitbit and PSG. The Fitbit's sleep staging algorithm may treat sleep episodes differently based on duration, leading to potential limitations in its accuracy.

The study also found that the Fitbit Charge 2 overestimated REM sleep latency and wakefulness after sleep onset (WASO). The accuracy of REM sleep and WASO was higher than that of other sleep stages, such as N1+N2 and N3. Additionally, the Fitbit heart rate displayed a small underestimation of 0.9 beats per minute and struggled to capture sudden heart rate changes due to its lower time resolution compared to PSG.

In conclusion, the Fitbit Charge 2 can provide reasonably accurate sleep and heart rate estimates under naturalistic conditions. However, there are some limitations to the precision of quantifying individual sleep episodes and capturing sudden heart rate changes.

Fitbit Charge 2 sleep tracking limitations

The Fitbit Charge 2 has shown promise for clinical sleep tracking, but it does have some limitations.

Firstly, the Fitbit Charge 2's sleep staging algorithm treats REM and deep sleep states differently if they are shorter than 4.5 minutes, which introduces a non-biological discontinuity and indicates potential limitations in the tracker's staging algorithm. This means that the Fitbit Charge 2 may not accurately capture short sleep stages, which can be an issue for people with known sleep variances, such as adolescents and the elderly.

Secondly, the Fitbit Charge 2 overestimates REM sleep latency and wakefulness after sleep onset by several minutes. This could impact the precision of quantifying individual sleep episodes, especially for those with sleep disturbances.

Additionally, the Fitbit Charge 2 has a limited capability to capture sudden changes in heart rate due to its lower time resolution compared to polysomnography (PSG). This results in a small underestimation of heart rate, which is more pronounced during wakefulness and light sleep stages.

Moreover, the Fitbit Charge 2 may not accurately differentiate between restlessness and awake time. Some users have reported that the device registers periods of wakefulness as "restless" even when they are actively using their arms, such as when reading a book or using their phone. This limitation can affect the overall accuracy of sleep tracking, especially if the user has a habit of spending time in bed while awake.

Lastly, the Fitbit Charge 2's sleep tracking feature is not intended to diagnose or treat any medical condition and should not be solely relied on for medical purposes. It is recommended to consult a healthcare professional for specific questions or concerns related to one's health and sleep patterns.

Fitbit Charge 2 sleep tracking for clinical purposes

Sleep tracking is an important tool for clinical and research purposes. Traditionally, polysomnography (PSG) has been the gold standard for sleep tracking, as it can accurately measure sleep-wake state patterns and the composition of sleep stages. However, this method can be costly and intrusive, requiring patients to be monitored in a sleep laboratory.

The Fitbit Charge 2 is a multisensor fitness tracker that offers a less expensive and less intrusive option for sleep tracking. It can estimate sleep quality in a home environment, providing data on sleep patterns and heart rate. While it is not intended to diagnose or treat medical conditions, the Fitbit Charge 2 has shown promise for clinical sleep tracking, especially in populations with consistent sleep patterns.

A study was conducted to validate the Fitbit Charge 2 against PSG in a shift-work population of police officers and paramedics. The Fitbit Charge 2 demonstrated reasonably accurate mean values of sleep and heart rate estimates, with a strong accuracy in detecting sleep stages. However, it struggled to differentiate between REM and deep sleep states of less than 4.5 minutes, and it overestimated REM sleep latency and wakefulness after sleep onset.

The Fitbit Charge 2 also faced challenges in detecting deep sleep, with an accuracy of only .49 when compared to PSG. Additionally, it had difficulty specifying when a participant was in a waking state. These limitations indicate that the Fitbit Charge 2 may not be suitable for sleep tracking in populations with known sleep variances, such as adolescents and the elderly.

To improve the clinical utility of the Fitbit Charge 2 for sleep tracking, further enhancements are required. Open-source algorithms, raw data access, and the ability to blind participants to their sleep data could potentially increase the value of this consumer-grade multisensor wearable.

Fitbit Charge 2 sleep tracking compared to other devices

The Fitbit Charge 2 is a multisensor fitness tracker that can estimate sleep quality. It does this by looking at how much you move and how often your heart beats to infer whether you are awake, in a deep sleep, a light slumber, or dreaming. Fitbit Charge 2's sleep staging algorithm probably treats REM and deep sleep states of less than 4.5 minutes differently from sleep stages that exceed this duration. This is one of the potential limitations of the tracker's staging algorithm.

Fitbit Charge 2 was found to be reasonably accurate in estimating sleep and heart rate (HR) in shift workers, according to a study. However, the study also found that the Fitbit Charge 2 overestimated total sleep time by 9 minutes and underestimated sleep onset latency by 4 minutes. It also overestimated light sleep by 34 minutes and underestimated deep sleep by 24 minutes compared to the PSG-derived sleep stages.

Another study compared the Fitbit Charge 2 with the Fitbit Charge 3 and actigraphy devices. The study found that the Fitbit Charge 2 tended to overestimate total sleep time (by 9-60 minutes) and underestimate WASO (by 5.5-36 minutes) compared to PSG. In contrast, the Fitbit Charge 3 tended to underestimate total sleep time (by 6-12 minutes) and overestimate WASO (by 9-37 minutes).

The Fitbit Charge 2 has also been compared to the Fitbit Versa, with similar results. Both devices were found to give slightly different readings, but the overall picture was the same. This suggests that the Fitbit Charge 2 is consistent with other Fitbit devices in tracking sleep.

In summary, the Fitbit Charge 2 sleep tracking appears to be reasonably accurate in estimating sleep quality, especially when compared to other Fitbit devices. However, there are some limitations to the device's staging algorithm, and it may not be as accurate as traditional sleep research techniques such as polysomnography (PSG) and actigraphy.

Fitbit Charge 2 sleep tracking settings

The Fitbit Charge 2 can track your sleep patterns, including your sleep stages and heart rate. It can also provide a detailed sleep breakdown, or sleep profile, which is available on the first of every month. This includes 10 monthly metrics and a sleep animal that characterises your long-term sleep behaviours.

To get your sleep profile, you must have a Fitbit Premium subscription and a compatible device. While the Fitbit Charge 2 can track sleep patterns, it is not listed as a compatible device for the sleep profile feature.

To track your sleep with the Fitbit Charge 2, you must wear your device during sleep. Fitbit recommends charging your device to at least 40% before going to bed, as the sleep tracking feature can impact your device's battery. The device will not be able to identify whether snoring comes from you, someone sleeping nearby, or a pet. Your sleep must be at least 3 hours long to receive a Snore & Noise Report. The device only uses the microphone to register noise level and snores and does not save any audio recordings.

To accurately track your sleep, make sure your device is positioned higher on your wrist, about 2-3 finger widths above the wrist bone. The band should feel secure but not too tight. You can also adjust your sleep sensitivity setting—if it is set to Sensitive, change it to Normal.

The Fitbit Charge 2 will also track your stats more accurately if it knows whether you are wearing your tracker on your dominant or non-dominant wrist. You can adjust this in the Handedness settings.

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