Usage going up on it's own

[#1]

Hi,
Having exceeded my usage twice now I have realised that over the course of the month my day time and over night usage has been used even when all PCs have been turned off.  I have also today realised that my overnigth usage has gone up to 851mb before midnight.  How and why is this happening?
I have taken some screenshots of the problem. 
#1 Defined overnight usage hours:

#2 Overnight usage before midnight:

I am also getting quite high pings at the moment anyone else having this problem?
All the best,
Charlie.

[#2]

If you have a wireless network, perhaps most likely reason is a neighbour is probably connecting to your network and using it without your knowledge.
Is wifi security turned on at the router?
Are you using WPA2-PSK encryption?
Do you have a strong encryption password?

[#3]

Cheers for the reply, I don't think that is the case, we have a decent passcode, I will change it for luck anyway. 
However, that still doesn't explain the overnight usage already being used at 7.00pm and not after midnight.
Charlie.

[#4]

fyi, the overnight usage shown for 02 Nov includes the period from midnight to 8am this morning, and not tonight which would be for 03 Nov.

[#5]

Have you got any smartphones (iPhones, Android or BlackBerry) or tablets like an iPad connected to your wi-fi? Some consoles such as the PS3 can also be set to "wake" and check for software updates during the night.
Plugging an iPhone in overnight will start the iCloud backup and that could be a few GB

[#6]

Quote from: zzZzz

fyi, the overnight usage shown for 02 Nov includes the period from midnight to 8am this morning, and not tonight which would be for 03 Nov.

Ah of course that makes complete sense.
I have no phones that connect to the wireless.
Charlie.

[#7]

plus to look at the free period usage you have to click on the useage this month button, that then give two lines of info