Why do go apps to sleep?
Sleeping allows apps to be suspended so the phone has less work to do. This is mostly to preserve battery life.
How does sleeping prevent battery drain?
When an app is put to sleep, the phone stops processing its requests. It does not listen for notifications that come in from the internet, or anything at all. In short, the application deactivated and only reactivated when you use it again.
How to prevent app from going to sleep?
Sleep management differs between vendors. Head over to Don’t Kill My App to get most up-to-date information on how to prevent sleep.