A limitation, feature or inconvenience of Windows (XP, Server 2003, Windows Vista, or Windows Server 2008) is that it cannot hibernate if it has more than 4 GB of RAM memory.  This is not a bug but by design, as Microsoft disables hibernation support on PC with more than 4GB physical memory because benchmark tests by Microsoft show that performance is poor on a computer that has more than 4 GB of memory with hibernation enabled, which requires as much disk space as RAM for the hibernation file, c:hiberfil.sys.

So if you have more than 4GB of RAM installed you won’t see the hibernation tab in the Power Options Properties dialog box.

More information from Microsoft – KB888575.