[MacNNUK]

I've noticed that my Broadband usage seems to be creeping up quite a lot, more than I would expect.

Before, on Orange UK BB USB Dongle, I would disconnect before putting iMac computer to sleep, so no extra downloading.
Or sleep would disconnect the USB dongle anyway.

Using the built in Airport wi-fi to my wireless router, with UK ISP, I noticed my usage was going up even when iMac was in sleep mode.

I think I have found the reason, sometimes when I click on a webpage link, it also opens up another browser window behind the main one, usually just static pictures, but the last time I checked the background window had a flash movie going on in it, which seemed to be constant. 100mb added to my usage over an hour or so.

I hadn't noticed the extra window (with constant movie) and closed it before sleeping the iMac.
I think this is where my usage is going up, when unattended.

Has anyone else had the same problem, ie do you continue to download wi-fi even when computer is sleeping ?
Would this also happen if I connected via ethernet cable ?

It looks like I will have to manually close all background windows, or quit browser, or turn Airport OFF before sleep.

[Waragainstsleep]

A sleeping Mac should not be downloading anything. Newer Macs support remote wake over Airport but this is usually restricted to listening for traffic on the local LAN, it shouldn't be sending anything over the internet. A flash movie playing on a loop shouldn't be sending anything either.

[Big Mac]

A sleeping Mac is incapable of downloading anything. All of its components are either off or in a very low power state.

[Chito]

More than likely someone else is connecting to your wireless router.

[Spheric Harlot]

Do you perhaps have an iPhone or an iPod touch logged in to the wireless network?

What kind of encryption are you using for the wireless LAN?

What devices to the router's log files show?