Darkening and Restoring the Display
Depending on your screen timeout setting, the display automatically turns off during periods of inactivity. In addition to waiting for this timeout to occur, you can manually darken the display to conserve the battery or maintain privacy by pressing the Power button on the right side of the phone.
Restore a Dark Display
Press the Power button on the right side of the phone to make the lock screen appear. (The lock screen also appears when you turn on the phone.)
Swipe in any direction to dismiss the lock screen.
Set the Screen Timeout Interval
On the Home screen, tap Apps, followed by Settings.
In the Device section of Settings, tap the Display icon.
Tap Screen Timeout.
In the Screen Timeout dialog box, select a new timeout interval or tap the Cancel button to retain the current setting.