In previous versions of Mac OS X, the OS would wait for at least one minute of disk inactivity before putting the system to sleep. This decision could be vetoed at the last moment by any application that preferred the system to stay awake. In Mountain Lion and Mavericks, OS X no longer pays attention to disk activity when deciding if it's OK to put the system to sleep.
Please make sure your Mac is not set to go to sleep during a scheduled or on-demand scan or backup. Sleep mode may prevent these tasks from completing as expected.
You can find more information about changing your Mac OS X Energy Saver Preferences here: