clean-buffer-list-kill-regexps is a variable defined in `midnight.el'.
Its value is
("^\\*Man ")

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

You can customize this variable.