Android 4.1+: Doze is one of the best new features in Android Marshmallow, which stops apps from using the network when your phone is idle. But if your phone still doesn't have Marhsmallow, this app will bring the feature to older phones.
Once installed and activated, Doze creates a VPN, which the developer says is more a barrier to block data flow rather than an actual VPN. Doze kicks in once the screen is switched off, but you can whitelist certain apps to use data. Considering that data connections drain your battery quickly, it's a good way of saving power.
You can also set Doze to auto-quit when you're connected to Wi-Fi or a charger. It's a simple app that brings one of Marshmallow's coolest features to older Android devices.