How do I change the Hibernate logging level?

Hibernate uses SLF4j as it's logging API. Looking at this API I can't find any way to adjust the logging level programatically.

So in my opinion you have to use the underlying runtime logging system directly. This depends on what you have configured. When using log4j this would be something like

import org.apache.log4j.Logger;
import org.apache.log4j.Level;

...    

Logger log = Logger.getLogger("org.hibernate.SQL");
log.setLevel(Level.DEBUG);

But remember that going this way your code is dependent on the underlying logging implementation.