Open IIS, right click the application pool that your portal's using, choose "Property".
In the Performance tab, uncheck the checkbox "Shutdown Worker Process in XX minutes".
Otherwise, if you don't want to do so, you can install Smarter Ping directly on your server to keep the DNN application alive. I'm not sure if the program still exists. If you can't find it, email me at lehoangduc1990 [at] pijaja.com , I'll send you the file.
HTH,
Duc