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Is this caused by low battery? I have an Asus UX31A, and I left it unattended. As usual, in 5 minutes, it went to sleep mode with the lid still open. I came back a couple hours later, and I noticed the laptop was off. I was still able to turn it on by manually pressing the power button (battery was at 2% though), but I don't understand why it shut off. If it was in sleep mode and the battery was near depletion, then wouldn't that result in a "Windows has recovered from an unexpected shutdown" upon boot? Does Windows have a mechanism to wake from sleep from a super critical battery level to shut down all programs and power down the system completely? I noticed that upon booting, firefox didn't give a fit about improper shutdown.

Here are my power settings btw,

power settings

powercfg -h off has been ran, so hibernation is off. And here is the event log:

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Windows can do some interesting things with sleep. I believe it can be configured to wake and then go into hibernate mode (which would avoid giving you errors from an unexpected shutdown). It doesn't look like your machine is configured that way (I'd expect to see "sleep after" x minutes, and then "hibernate after" x minutes, or perhaps "Critical battery action" as "hibernate").

My guess (although the corresponding option in your power settings output you includes is missing for some reason) is that your machine has "hybrid sleep" enabled, which saves the contents of RAM to your hard drive at sleep time and then sleeps as normal. If a power loss occurs, then the contents of RAM are restored from the hard drive, just as if you had originally hibernated instead of put the machine to sleep.

From Microsoft:

"Hybrid sleep is designed primarily for desktop computers. Hybrid sleep is a combination of sleep and hibernate—it puts any open documents and programs in memory and on your hard disk, and then puts your computer into a low-power state so that you can quickly resume your work. That way, if a power failure occurs, Windows can restore your work from your hard disk. When hybrid sleep is turned on, putting your computer into sleep automatically puts your computer into hybrid sleep. Hybrid sleep is typically turned on by default on desktop computers." source:

Perhaps that option is missing because this feature is primarily for desktops, or because the manufacturer (if you haven't clean installed Windows after purchasing the machine) has somehow removed it from that list. It could still be enabled, but not customizable for you.

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