This might be useful information for people having the same problem as I had : Windows 7 waking up from sleep mode without any apparent reason, i.e. no keyboard activity and no mouse activity.
It turns out that the Razer Mamba driver package installs a “virtual keyboard” (probably for the Mamba’s docking station?), which wakes up the computer from sleep for no reason.
The solution is to deactivate this device’s ability to wake the computer from sleep, from within the device manager. Look at your end and you’ll probably also see two “keyboard” type devices. The installation date should give you a hint on what is what!