Answer Description
Sleep, Hibernation, and Stand By are the only true System Power States. Of the three, only hibernation provides no power to memory, and saves memory contents to the hard drive to be loaded back into memory when the computer wakes up.
Wikipedia
Hibernation (or suspend to disk) in computing is powering down a computer while retaining its state. Upon hibernation, the computer saves the contents of its random access memory (RAM) to a hard disk or other non-volatile storage. Upon resumption, the computer is exactly as it was before entering hibernation. Like a powered-down system, the power source from a system in hibernation can usually be removed without severe state loss …
Hibernation (computing) - Wikipedia, the free encyclopedia