User Session Auditing

Is there a way to know which USER logged in and when ?

Yes, there is. For Desktop, EE and other versions you're looking for a file called security.log in the same directory where your other logs are. (Generally something like /var/log/neo4j/ or $INSTALL_DIRECTORY/logs This isn't yet available for AuraDB though. See this documentation entry for options/file locations and so on.