Sleeping just means not currently running a query. It means the connection is open, but there are no pending requests. Runnable is not a state for a SQL connection. Sleeping = waiting for something to do Running = currently executing a query Suspended = waiting for a resource (lock, latch, etc)
Typically lots of sleeping connections indicates an app that's either using a connected architecture (open connection and never close it), or one that's careless about closing connections once done.