Why does my server go to sleep?
Sleeping is how the free tier stays free, and what happens to your world.
When nobody has been online for a few minutes, the server saves its world and stops. The hardware it was using goes back to the pool so someone else can play.
Your world is not deleted. It is saved first, then uploaded, and restored when the server next starts.
You can change how long it waits — 5 minutes up to an hour — under Configuration on the server page. There is no "never": a server that never sleeps holds a slot that other people are queueing for.