This TV has an inbuilt 'switch off' feature which has been included to preserve the OLED panel and reduce the risk of irreparable screen damage and screen burn.
The TV will automatically switch off after extended periods of use, approximately 4 hours.
It is a common feature for this technology and is a standard in which most other brands include with their TV's.
In order to assist with prolonging the life of the unit, this feature cannot be turned off.