Well, if not battery/sleep/energy, then I would perhaps be thinking screen.
If cm lite is Clean Master Lite, then this could be screen-related, Night screen literally is screen-related.
How probable is it that you would have the same problem on 2 phones running different Android versions (if you were not running Android 9.0 in the form of LineageOS 16.0 on your Fairphone 2) on different hardware, when other people don’t have this problem? But you are roughly using the same set of Apps.
You might consider uninstalling Apps one by one to find the culprit … or doing a factory reset and adding your Apps one by one.