Sleep tracking draining Apple Watch battery

I have an Apple Watch Series 4, which I got secondhand in almost brand new condition about a year ago. I have it on the most recent Watch OS update. I use it in Sleep Mode to track my sleep overnight, and I charge it up to 100% before bed each night. When I wake up in the morning about ten hours later, it has gone down to around 30%, sometimes even as low as 10%. I don’t understand how tracking my sleep uses significantly more watch battery than during the day when I wear it all the time and track exercises etc. I’ve seen a few people having similar problems, does anyone have any idea if I can do something to fix this? Obviously I could just not track my sleep but I find it very useful.