Sleep is the mode in which many of the components off of the computer but not the RAM. It continues to consume energy. But start the computer from this mode is the fastest of all the three considered. But the energy in this mode, the computer consumes a maximum of 3.5 to 4 watts.
Hibernate is more developed and deep sleep mode. All memory contents are dumped to the hard drive, so memory de-energized completely. Power is only required for the functions Wake-on-USB, Wake-on-LAN. These features allow the computer to Wake up from the movement of the mouse, or network activity. So it is necessary to feed the USB ports (to save power) and the network card. The computer consumes in this mode less than 2 watts, but the consumption can be reduced almost to zero by the disabling in bios of these functions bodylock. Still most of the users Wake the computer by pressing the power button.
A complete shutdown. It would seem here to spend energy on that, but open the case. Will definitely find burning the led on the Board or on the network card. A small loss of energy is in this mode.