Using up my limit overnight - why/ How?

[#1]

Hi
As a new Plusnet user I have received a notification that I am nearing my limit.  However, when checking my usage breakdown I see that every night a quantity of my allowance is being used and don't understand why.
can you help me on this.  Just a by the way on this both myself and my husband use an iPhone is this relevant for email downloads overnight, if so how can it be so much used every night, quite a few hundred mb per night.
Thanks

[#2]

Between midnight and 8 am all traffic is free and does not come off your allowance.
But if you are not using it, is your router secure ?

[#3]

Are your iPhones on iOS5 or later? If so and you plug them in to charge overnight they will auto backup to iCloud every night.
Alex