Mystery of Hibernation in Windows Vista

My system drive is running low in space, so I decided to do a disk cleanup. There are files called "Hibernation File Clearer", which takes over 2.74G. I selected and did the clean. This action caused HIBERNATION choice disappeared from Vista everywhere: menu, control panel, etc.

To turn on hibernate in Windows Vista, I have to:
1. Run cmd.exe as adminstrator by holding the shift and right click the cmd.exe and choose Run As Administrator.
2. Type the "powercfg /hibernate on".