How to disable "Sleep mode starts soon" alert?

So on my second night with the watch, I waited with my camera ready to photograph the alert at 8:30pm, however, there was no alert on the second night. 

I spent a good amount of time messing with settings yesterday and never found one related to this issue. It’s possible I unknowingly changed something that affected the alert .

I also researched online and found nothing. 

Maybe the alert is only for the first night you own the watch. 

I’d still like to know the answer. 

I think it's only for the first night.