If this answer does not work in Ubuntu 13.10, see here for an alternative answer

Hibernation was disabled on 12.04 for machines that are not certified with Ubuntu.

In order to enable hibernation you need to test whether it works correctly by running sudo pm-hibernate in a terminal. The system will try to hibernate. If you are able to start the system again then you are more or less safe to add an override.

To do so, start editing:

sudo nano /etc/polkit-1/localauthority/50-local.d/com.ubuntu.enable-hibernate.pkla

Fill it with this:

[Re-enable hibernate by default]

Identity=unix-user:*

Action=org.freedesktop.upower.hibernate

ResultActive=yes

Or in 14.04 and later:

[Re-enable hibernate by default for login1]

  Identity=unix-user:*

  Action=org.freedesktop.login1.hibernate

  ResultActive=yes



[Re-enable hibernate for multiple users by default in logind]

  Identity=unix-user:*

  Action=org.freedesktop.login1.hibernate-multiple-sessions

  ResultActive=yes

Save by pressing Ctrl-O then enter and then exit nano by pressing Ctrl-X.

Restart and hibernation is back!

Or run killall unity-panel-service to just reset the menu.

Some users will then need to run sudo update-grub to get the hibernate option to be available in the power menu. Some users may also have to at least log out then log in to get it to appear in the (upper right) power menu.