Talkative Little Peripherals

I installed Cacti last week. Cacti graphs, among other things, transfer statistics from SNMP-capable devices such as my Apple Airport Extreme router.

These graphs that Cacti makes show me wifi use as well as my total internet use. Generally this is more vaguely interesting than useful. Generally.

So, riddle me this. From 6pm to 7am every day, wireless traffic holds steady at 60k/sec. The only other wifi device that's consistently available is the Apple Airport Express (which is an unlikely culprit), and if the router is at fault, it isn't necessarily talking to any other device; it could just be sending data nowhere. Still ... I'd like to know. And 6pm is right around the corner.