You may receive the following error in dnn admin account -> admin -> event viewer (for DNN4.0 or above) or log viewer (for earlier version of DNN):
Application Shutting Down Shutdown Details: The AppDomain shut down because of the hosting environment
The reason for this error is that, when your dnn site is idle for about 20 minutes, IIS would shutdown the application pool your site located in order to maintain a healthy server environment.
The DNN will then logged it as Application Shutting Down. But once another visitor visit your site, IIS will start the application and your dnn site will be recompiled to serve the visitor.