Fortunately, I've kept my original S0 for this exact reason. It isn't worth much (maybe $25 at this point), so I figured I might as well keep it for sleep and also to use when I do yard work. It still tracks activity and sleep like a champ. Battery life is still good.
I did find sleeping with the watch slightly annoying though because the light would disturb my sleep at times when it would turn on and the watch was close to my face. Once I enabled tap to wake instead raise to wake... this solved my issue.
Of course, the watch's days are numbered as better apps come out that require WatchOS 5. Once that happens, I'll probably just keep the last version I have when I get a new watch in the future. For now, its S4 & S0 for me.