Prevent Windows 7 PC from sleeping while a download is in progress
Check this tool: Insomnia which prevent the computer from going into sleep mode while the program is open.
It is the duty of the downloading program to reset the system idle timer by calling SetThreadExecutionState with ES_SYSTEM_REQUIRED.
Maybe you are lucky filing a bug/feature request at google code
FlashGet has the option to put the PC in sleep mode after all downloads complete.