The LED light on Sony Xperia Z3 informs you about new notifications, like incoming calls or new text messages. To disable / enable notification light on Xperia Z3 and Xperia Z3 compact, do as follows:
1. Open Settings and under the Device settings, tap the Display.
2. Scroll down and mark the "Notification Light" option to enable the feature. (If you want to disable notification light on Xperia Z3, simply unmark the option)
Note that, regardless of the Notification Light status, it turns on when you are running low on the battery