Sony SGPT21 Series Help Manual page 156

The screen disappears/darkens unexpectedly.
The screen is turned off automatically after a certain period of time to reduce the battery consumption.
The screen turns on again when you push the power button.
When you want to change the time it takes to turn off the screen, tap [ Apps] - [ Settings] - [
Screen] - [Timeout] in that order to set the time.
If you set the time to a longer interval, the battery will be consumed faster.