Many are fans of particular OEM skins and their built-in features. One of the features available in stock Xperia ROMs is a Battery Stamina Mode, which was introduced to help users make the most of their batteries.
In this mode, all connectivity except GSM is disabled. This also disables your device’s LED lights. It works pretty well, but many find that disabling the LED is unnecessary. After all, without an LED, it’s rather easy to miss a call or message.
Now thanks to an Xposed Framework module by XDA Senior Member itandy, you can re-enable your LED leven when in Battery Stamina Mode. As you would expect, you need to be rooted and have Xposed Framework installed to get this going. And since this is an Xposed module, you can easily disable this when you want to go back to standard Battery Stamina Mode.
You can find the module in the original thread.