iTunes can automatically back up your iPhone each night if you satisfy the criteria. They are as follows:
Devices must be locked
Devices must be powered
There must be a Wi-Fi connection on the device
You must connect your PC or Mac to the same Wi-Fi network if your phone backs up to iTunes over Wi-Fi.
Creating a wireless backup is also possible when the device is connected to the same network. Open iTunes and wait until the device icon appears on the top navigation bar. To create a wireless backup, click on it, then click Back Up Now.