clean-buffer-list-kill-buffer-names is a variable defined in `midnight.el'.
Its value is

("*Help*" "*Apropos*" "*Buffer List*" "*Compile-Log*" "*info*" "*vc*" "*vc-diff*" "*diff*")

List of strings saying which buffers will be killed at midnight.
Buffers with names in this list, which were not displayed in the last
`clean-buffer-list-delay-special' seconds, are killed by `clean-buffer-list'
when is it in `midnight-hook'.
If a member of the list is a cons, its `car' is the name and its `cdr' is
the number of seconds to use instead of `clean-buffer-list-delay-special'.
See also `clean-buffer-list-kill-regexps',
`clean-buffer-list-kill-never-regexps' and

You can customize this variable.