Bests & Worsts Reviews from Amazon

I want to have a wearable to measure heart rate, SpO2, and track sleep. There are not many wearables for spO2 in the market, and Garmin Vivosmart 4 is one of them. The heart rate measure is quite accurate. SpO2 measure is also accurate when I compare the readings to my another two pulse oximeters. Unfortunately, SpO2 tracking is limited to 4 hours. It could be better if I have an option for 8 hours. HRV (Heart rate variability) is a very important heart parameter, which could be estimated from a pulse oximeter (there are some study available). It is much useful than using HRV in stress tracking and body battery monitor by Garmin. Both stress tracking (a rescaled HRV using user's historical data) and body battery monitor (a function of HRV, heart rate, quality of sleep, etc.) licensed by FirstBeat may mean nothing. For example, I can have too kinds of HRV measures, one in exercise and one in rest. When the HRV value is rescaled between 0 to 100 based on my historical data, the stress tracking result completely differs from the rescaled HRV when I do not have any exercise. The rescaled HRV concept is useless, and a subjective number. One cannot compare the stress measured to other people. Garmin (actually FirstBeat) makes you feel better by give you better reading if you have really bad HRVs, because actual HRV number is normalized based on your own reading. The fix for this is to display the real HRV value. The true HRV is objective and comparable number. It means much more than the pressure concept used by Garmin or FirstBeat. Vivosmart 4's sleep function seems not accurate. I know it is tough to measure sleep time accurately using one sensor. Unfortunately, the sleep time tracked by Garmin is way off. It could miss by several hours. When this inflated number is included into body battery monitor, what does the body battery mean? One major technical issue in sleep tracking is: Deep, light and awake sleep time are not increasing number from zero overtime. They can randomly vary during sleeping and after sleeping! When I was awake during the night, I saw my deep sleep time is about 2 hours and made a screen shot. The deep sleep time became 16 minutes in the morning, and then become one hour in the afternoon. I hope Garmin can fix this. Currently, it does not tell me anything about quality of sleep. I will continue using Garmin Vivosmart 4 because of heart rate and SpO2 tracking. [Update on SpO2 tracking after using Vivosmart 4 for one month] : instant measure of SpO2 from Vivosmart 4 could be accurate assuming it has a tight fit to the wrist. However continuous measuring of SpO2 may not be accurate. My tracking results in the past are mixed. The SpO2 readings from Vivosmart 4 fluctuated more during night. It may under-estimate SpO2 average by 5 units (e.g., 92 vs. 97) comparing to the result from my pulse oximeter. It is better to use other devices for continuous SpO2 tracking when more accuracy and better reports for SpO2 are needed. Update: Change the review to one star, because of completely inaccurate sleeping tracking and bad SpO2 reading during night.