Sounds to me like it's not actually in sleep mode.
What happens when the Mac goes to sleep (not just into idle mode or display sleep)?
On all Macs:
The microprocessor goes into a low-power mode
Video output is turned off, and a connected display may turn off as well, or enter its own >idle state
Apple-supplied hard disks spin down; third-party hard disks may spin down
On portable Macs:
The Ethernet port turns off, if applicable (see note below)
Expansion card slots turn off
The built-in modem, if present, turns off (see note below)
An AirPort card, if present, turns off (see note below)
The USB connection only responds to the power key on an external keyboard (see below)
The optical media drive spins down
Audio input and output turns off
Keyboard illumination, if a feature of your portable computer, turns off
About sleep mode