iPhone is not backing up to iCloud My iPhone 6S on IOS 9.3.2 has been set in iTunes so that it automatically backs up to iCloud. However it only backs up if I manually choose to do so from the iPhone. 

The phone is charged at night and has a working WiFi connection, it is running a sleep monitor app.

Why is it not backing up and what do I need to do to make it do so?
The reason behind is because your device is not locked.
As stated in this Apple document that in order to "Automatically back up with iCloud Backup" 


Make sure that iCloud Backup is turned on in Settings > iCloud > Backup
Connect your device to a power source
Connect your device to a Wi-Fi network
Make sure that your device's screen is locked
Check that you have enough available space in iCloud for the backup