Yeah, when I used to actually bother checking it, because I'd be watching TV for a couple of hours before going to bed, it would register that I'd slept from 8pm to 7am. This despite getting up off the sofa, going up the stairs, getting washed and ready for bed. I only really bought it as a means to silently wake me up for work, by way of its vibration function, to avoid waking the missus up. I also set an arbitrary target of 8000 steps per day which I pretty much never reach these days. That's working from home for you!
4 vs 5, easier charging, a barometer, and a few more activities... https://www.androidcentral.com/xiaomi-mi-band-5-vs-mi-band-4
I wouldn't say it was very reliable though. I'm up reading most nights and because there is little movement it registers it as sleep. Handy as an overall impression though.
Yes. Had my 4 for ages and the battery still lasts just under a month. I do t use it for much more than a watch and notifications to be honest.