I'm using an AMD Phenom II desktop running Windows 7 that I custom built. I put the system to sleep (not hibernate) then I accidentally shut off the power supply that it was connected to as well, so the system was not being supplied any power. My understanding of sleep is that it needs a little power since it does not save any information to disk. However, when I turned on the system several hours later, it still woke up from sleep and all the windows I had open were restored just as they were before.

How is this possible?!? :O


It's hybrid sleep, a combination of hibernation and sleep. It saves memory to disk, and then enters sleep (the low power state). If power is maintained, when you turn the computer back on, it simply wakes from sleep. If power is lost, it can recover, as if the computer was hibernated.

  • Then why is it faster to go into hybrid sleep than normal hibernation mode? – Phenom Dec 30 '09 at 13:16
  • Is it really faster? It may appear longer to watch the on-screen "Hibernating" indicator, and then the time it takes to actually shut down the power; than for the screen to go blank, quietly save to disk, and slip into sleep. Another possibility is that it sleeps immediately, but then only hibernates after "a little while", when it is seems that you are not going to wake the computer (e.g. because you accidentally let it go to sleep). You might try watching your computer to see if that is the case. – Ken Dec 30 '09 at 14:12
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    Hybrid sleep appears faster because it sleeps first, then carries out the Hibernate after. Think of Hybrid Sleep like Async I/O and Hibernate as Sync I/O. Hibernate needs to confirm memory is written out before it acknowledges that it is complete, Hybrid sleep can sleep first, then spool out the Hibernate to disk while sleeping. – Darren Hall Dec 30 '09 at 17:36

Because starting with Vista, sleep got a little smarter. It now also writes everything to disk, meaning you can cut the power just fine. It's pretty neat, and has saved me a good few times.

  • I wonder how that is different from hibernate? Why not call it hibernate? can you update with some points of difference between the two ways? – nik Dec 30 '09 at 12:07
  • Then how come it still takes longer for the system to go into hibernate? – Phenom Dec 30 '09 at 12:07
  • It's faster to come out of hibernation, and faster to go into sleep - apart from that I believe they are now very similar. – Phoshi Dec 30 '09 at 12:07
  • I have no idea, but consider that when you go into sleep mode, hibernate.sys seems to appear with the contents of one's RAM, this seems the most likely explanation. – Phoshi Dec 30 '09 at 12:08

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