One last several months, the other perhaps overnight...
Hence MS using the terms.
Hibernate (cut and paste to save typing):
Hibernation is when your computer system shuts down to save power, but first saves everything in memory on your hard disk. When restarted, your desktop is restored exactly as you left it. Hibernation is used to save power if or when you are away from the system for an extended time while working.
In the case of Sleep it's really another way of expressing Standby; when you restart the system by using the mouse or a key stroke, you bring it out of a "Sleep" mode.