Hibernation mode cuts off the power while saving your open programs and data. In the process, it eats up needed disk space.
Sleep mode simply shuts down power to the display and drives, while hibernation mode cuts off the power while saving all your open programs and data to disk, so you can resume right where you left off. While this is convenient, it also eats up valuable gigabytes of disk space. That’s because, by default, Windows 7 creates a hidden file called hiberfil.sys, which sits in the root of your C drive and takes up an amount of space equal to your RAM (usually 1 to 4GB).
To delete hiberfil.sys for good, you must:
1.Type “cmd” into the Start Menu search box and hit Enter. A command prompt appears.
2.At the command prompt, type “powercfg.exe –h off” and hit Enter.
3.Reboot for changes to take effect.