Force iTunes update overnight using cron
You can use the following AppleScript to ask iTunes to update all your podcast feeds:

tell application "iTunes"
    updateAllPodcasts
end tell


This AppleScript can be performed from the command line through osascript:

osascript -e 'tell application "iTunes" to updateAllPodcasts'


You can use a range of tools to schedule your AppleScript to run each night. One option is to run the AppleScript as part of an iCal event: How to Use iCal to Run an AppleScript

Alternatively, you can create a launchd job ticket and place it in your ~/Library/LaunchAgents folder.