Google Nexus 5X User Manual

I was just referring to recent memory of yours, maybe you installed some battery saving app. Anyway, you can go to Settings > Display > Screen timeout (or Sleep) > Choose long times. Some phones have option "Never".

With regards to battery heating up, this happened to me on a newly bought phone. What I just did was uninstall some of my apps, specially those that keep syncing stuff (or you can just set them to manual sync). Hope that helps.

I don't keep a record of when and what apps I install.  Maybe I need to do that in the future.  Consequently, I can't answer your question.  

However, I did do a factory reset ( based on a recommendation from Project fi) because an app was causing my battery to heat up , and the best thing to do was start from  scratch.  Prior to that, my phone was updated to Android 8.0.   

Hi Don. Did you recently install anything before that started happening?