System doesn't sleep when plugged into wired LAN
Using Ubuntu 12.04.2 (x86) on my Samsung notebook. When the system is operated with wireless networking, the power-saving features (e.g. sleeping after x minutes) work normally. However, I've noticed that when I have it plugged into wired LAN at home (which I use because the speed is so much better for copying files within my network), the system doesn't ever enter sleep.
How do I fix this? I suspect the system is receiving traffic over the LAN port that makes it think it should stay "awake."
Samsung Series 3 15.6" notebook (NP300-V5A-A02US)