I  have always wondered what the difference is between Sleep and Hibernate options provided by Windows Vista.

I’ve found out (more of confirmed), they have a common purpose: save the last known state of your computer and quickly load it when awaken.  When you resume work, the last known state is restored.  You don’t have to undergo the slow process of powering on your computer.

In Sleep (or Standby), the power supply to non-essential and non-critical components is withheld  and most system operations are shutdown and stopped. All data in physical memory (RAM) are kept in internal memory, and the whole system is placed in stand-by mode. It can be waken up and used almost immediately. In Sleep Mode, the power load is reduced significantly, not cut-off entirely. In case it runs out of power, the system will have to start again from a powered-off state.

Hibernate, on the other hand, will save all data from physical memory to the hard drive, then power off the computer. When a user uses the computer again, it will boot up and load the last state saved in the hard disk.  In Hibernate Mode, the power is cut-off entirely.

To review, the basic difference between Sleep and Hibernate is the way the computer saves the last known state and the consumption of power.  Sleep saves the last known state to internal memory and needs conservative power to maintain the last known state.  Hibernate saves the last known state to the hard drive and completely shuts down the computer.  Since Sleep does not shut down completely, it is faster when restoring the last known state of the computer but has the tendency to shut down eventually (but not immediately).

In my opinion Sleep and Hibernate are really just helpful when you are using a notebook/laptop.  I prefer to Hibernate to absolutely conserve power without having to undergo the cumbersome process of powering on the laptop (fresh boot).  I used to use Sleep but wondered why I lost power anyway after several hours.

Allegedly, Windows Vista has enhanced Sleep Mode so that notebooks will automatically Hibernate when battery power level is low. It also has built-in Hybrid Sleep Mode which ensure that system state is preserved when there is power lost.