If you would rather put your computer to sleep instead of in hibernation after being idle, Ubuntu's settings don't exactly make it easy. Here's how to change the preference manually from the Config Editor.
Whether it takes too long to wake up, you don't have enough space on your small drive to save your state, or hibernation doesn't work in Linux on your particular machine, you may decide that you'd rather your laptop sleep after being idle for a while. When on battery power, though, Ubuntu puts the system in hibernation by default after being idle, and you can't change this behavior in the preferences. However, with a bit of manual tweaking, you can make it go to sleep instead.
Open up a Terminal window or hit Alt+F2, type in gconf-editor and hit Enter. In the editor, navigate to /apps/gnome-power-manager/actions/ through the sidebar. There are two sleep type values, one for ac and one for battery. By default, Ubuntu puts your computer to sleep when plugged in, and hibernation when in battery mode (to save power). To change this, just double click on the value of sleep_type_battery (which should be hibernate), delete it, and type in suspend in its place. Now, your computer should slip into sleep instead of shutting down completely after the amount of time specified in your Power Management settings (accessible from System > Administration > Power Management).
Make Ubuntu Sleep Instead of Hibernate [Tips4Linux.com]