It's an interesting toy and PEM is visible in the data it collects. It tracks steps per day (imprecisely), heart rate, stress level, body battery, sleep phases. I'm not sure what measurements the stress level and body battery are based on. The body battery is the measure that seems most useful. When I clearly overdid it and then could not recover properly from the exertion, this was visible in the body battery. First, on the same day of the exertion, it showed a failure for the body battery to increase with rest and this is exactly how it felt to me. I tried to rest several times but could not relax and was not able to recover fully from the activity. The next morning, it showed the body battery had recharged to 67, whereas in the previous 3 days I had woken up with a charge of 100. And this also matches roughly how I felt. Listening to one's body is more accurate than the device I think, but it's still nice to see I'm not imagining things.
I also have a vivosmart4. (Currently not wearing it due to skin reaction with the band and I got a bump on my arm next to it.) It's a really nice device. It's far from perfect, e.g. it really struggles with tachycardia in combination with orthostatic hypotension. (So when in the shower it peaks but the measurement is often slower than counting actual pulse by hand, and when I have bad tachycardia under the shower, it can happen that while my manually felt and counted heart rate drops when sitting down for a bit, the heart rate monitor rises because it can better measure heart rate with rising blood pressure.) But like you it's a nice crude general observation tool. I find the body battery not very useful in daily numbers related to how I feel, but it does show trends accurately: weeks with higher scores are better than those with low ones. And a current antibiotics course is clearly visible: the body battery score dropped like it fell of a cliff. I can also observe PEM and bad crashes in the "stress" monitor. I find it actually measures physical stress a whole lot better than mental stress. Prominent PEM for me on the device can look like a block of high stress throughout the day while I'm actually lying down and reading. And of course you have a crude step count, which again is not so much use to me daily, but it does show trends.
I believe Heart Rate Variation (HRV), you can find it in the app. (Can't look it up now, my phone is charging and I'm Stuck Under Cat.)
The observation agrees with my pacing strategy that aims to arrive at the end of the day feeling like I have some spare energy (instead of being totally exhausted and/or wired and unable to relax).
I have the vivoactive 3 and you can add a HRV widget but you have to select it each time you want a reading. it doesn’t have the body battery function but it does have the body stress one. I have been using it for a few years now and it is really good at showing when your body shouldn’t be doing any activity. Sometimes I might feel not too bad and think I should do something useful ( housework , light gardening) but I check the body stress and it is really high so I take a day or two longer to relax. It then goes right back down. I have noticed that if I do take the time to relax until my body is ready, when I do then exert myself ( like walking upstairs) it doesn’t take as much of a toll as it normally would. If I am tired my heart rate normally goes racing and the body stress goes up to high 90s. But if I am rested this doesn’t happen. so I find it really helpful in working out what I can or can’t do when I think I am feeling okish and want to make the most of thinking I am ok to do normal activities.
Another interesting observation: during a PEM day, my stress level went down after a good meal. This is something that I've seen many times before I started using the device. The increase in blood sugar seems to help the body handle whatever it's going through.
On indicators of recovery from PEM according to the device. Looking at my "body battery" level in the morning after waking up, it took 5 days to reach a level of 97 (out of 100). During the previous days with PEM the level was much lower, suggesting a failure to recover during sleep. The stress level was also very low on awakening.
