wait_timeout kills session in SLEEP MODE or all connection ?`
Wait Timeout is a variable which represent the time in seconds that MySQL will wait and kill the session if it is in idle/sleep state.

Default value is 28000 secs. If you want to change the wait time value, you also need to change the interactive_timeout variable.

There will be no active sessions with this time interval. If your session is showing as active and some activity is happening so long, then there is issue with the session.