If you get the error "Authentication required before suspend" when the PC tries to suspend automatically, the actions below are required.
In the file "/usr/share/polkit-1/actions/org.freedesktop.login1.policy" search the section "<action id="org.freedesktop.login1.suspend">" and change the corresponding lines as below:
<defaults>
<allow_any>yes</allow_any>
<allow_inactive>yes</allow_inactive>
<allow_active>yes</allow_active>
</defaults>
An alternative solution, which is supposed to be permanent since the file above is updated occasionally, is given next.
Go to "/etc/polkit-1/localauthority/50-local.d/" and create a file named for instance "com.0.enable-automatic-suspend.pkla". Put the following text into the file:
[Enable automatic suspend]
Identity=unix-user:*
Action=org.freedesktop.login1.suspend
ResultAny=yes
Based on http://askubuntu.com/questions/785509/hibernating-from-cron/786225#786225
