Heroku is a platform used by commercial product developers and hobbyists alike. It is an ideal platform to proof of concept ideas before building sophisticated web apps on it. There are many users who use Heroku for that very purpose. Usually, these users have a bare minimum Heroku configuration. Specifically, they have just one web dyno running their application on the Heroku platform. Imagine 10,000 of these users trying to use the Heroku platform to host an application that isn't run regularly or run for short bursts of time. Well, given that Heroku is a very popular PaaS that hosts an enormous number of real-world web apps and a host of equally numerous hobby apps, Heroku has a way to optimize its use of dyno resources. Heroku shuts down every single web dyno app after an hour of inactivity.

If you check the application logs when the app is not active, they might look like this:

2013-11-30T08:23:09+00:00 heroku[web.1]: Idling
2013-11-30T08:23:17+00:00 heroku[web...