Thats a chest strap heart rate monitor though. I use it myself (when running or cycling) but I think it would be pretty uncomfortable to wear it while sleeping or throughout the day as a proxy for an activity monitor. 

On-topic: I use the Polar Vantage M watch myself. I like it in terms of tracking some relevant data (mainly runs & sleep) but not overdoing it. I don't know enough about the privacy preservation aspect. I guess if you do not create a Polar Flow account or never sync the wacht over Bluetooth / via the USB cable, then all the data will stay on the watch, so you should be good. And there's an 'airplane mode' and 'factory reset' on the watch. But I assume all this goes for most sportswatches without wifi connection, not just the Polar Vantage M.  

 yes it's not super comfortable to wear throughout the day, though tbh i never wear a watch as i find them uncomfortable too 😀 

that said, it looks like there's an open source utility to download the raw data from Polar watches through USB https://github.com/cmaion/polar 
i guess that should be safe to use privacy-wise 

 Ah cool, nice find man! I've been deep in the Polar Flow ecosystem for years so I have no illusions about my privacy there 😂 But still cool to play with, I'll see if I can get it to work later, thanks for sharing 🤩 @Gigi this looks like a pretty privacy-preserving option if you wanna go the Polar route 🥸