Chritoher Masto's Youtube video has some good tips and insights about this topic. However I think I found out how to utilize the official sleep function.
In the Nextion Editor I added a timer with the following properties:
- objname: sleep_timer
- tim: 15000 (15000 => 15 seconds)
- en: 1
In addition I added this to "Events" of the first page. Check Nextion Instruction Set for more.
// Enable wake on touch
thup=1
// set sleep on no touch
thsp=15
// sleep
sleep=1
This will send the display to sleep after 15 seconds.