wifi usage iphone/ipad whilst idle

My Home WIFI has peak and off peak times and a monthly quota. Off peak is from 2am to 8am.

I live in an ADSL1 area with slow access.

Until recently, say last two months, I had no difficulty keeping within my peak hour quota. Now I struggle and end up shaped for a day or so.

I have restricted my software and app updates to off peak , a great saving.

I switch off the iMac, Apple TV and 2 x PCs at end of day.

I use an iPhone 4S and iPad Air that have the latest iOS software.

I charge these devices overnight. They are reluctant to stay switched off whilst charging.

I plug them in to charger before switching off. By morning sometimes they are still off but mostly they have switched on again.

When the iPhone & iPad stay switched off, my usage is zero. so I know it's them but I do need to charge them.

Wifi data is being used in peak time when the devices are idle when I have retired to bed, say from 9pm onwards.

What is using this Wifi?

I have tried switching off background activities, diagnostic & usage stats and the like.

What am I missing?

As I write this, I am thinking that perhaps using airplane mode is the way to go.

Also could it be iCloud sync?

Words of wisdom would be received with much gratitude.