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My HP Pavilion laptop has a power light on the switch (a slide-and-release switch). This light is off when the computer is off or hibernated, constantly on when the power is on, and it blinks when the computer is suspended. Now, I figure that as far as the hardware is concerned there's no real difference between "hibernating" and "off". But why should the light blink when it's suspended?

What reason could a user have for differentiating between "off" (or "hibernating") and "suspend" states while the laptop is closed?

Most operating systems will automatically change to hibernation should the battery get too low, so user intervention for the sake of power management is unnecessary. I don't think there's any problem if the user flicks the power switch while the laptop is waking up from suspension (thinking that they need to manually wake it from hibernation), although avoiding this is about the only reason I can imagine. But if that's it, surely a better design would be to simply power up the machine from whatever state it's in upon opening.

(I hope this isn't too subjective... it's something I became curious about, and thought there might be an objective reason for it.)

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What is the difference between suspended and hibernating? –  Vitaly Mijiritsky Oct 21 '11 at 7:38
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@VitalyMijiritsky Suspending keeps the contents of the RAM safe but powers down most of the rest of the hardware, so that resuming is much faster but a tiny amount of power is used. Hibernation saves the RAM contents to the hard disk so that everything can be completely powered off, resuming is slower but no power at all is used. –  detly Oct 21 '11 at 8:08
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"What is the difference between suspended and hibernating?" - I think this actually a good answer to the question itself. Most users won't know either - so having a blinking power light isn't good design. –  PhillipW Oct 21 '11 at 9:33
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@PhillipW No, it can still be good design even when "most" users won't know. Who knows, maybe they start wondering and actually learn something in the process. And in any case, the users that do know and like that information, well, they are users too. "Good UX design" doesn't automatically apply only to the lowest common level of understanding. –  Ilari Kajaste Oct 27 '11 at 7:09
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@Mene Ah, true! I agree that the actual indication of a sleep state should definately not be attention-drawing. Blinking should only be used when there is something user is expected to be interested in (e.g. notification). I usually keep my laptop at sleep, and I've often had to physically move it away from my field of vision to hide that annoyingly distracting blinking! Yet, I definately want to be able to see whether it's on suspend or shut down (hibernate). –  Ilari Kajaste Jan 17 at 7:43
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I think you can break it down in two parts.

First, there is a difference between the hibernation and suspend (sleep) state. During hibernation the computer saves its state on the hard drive and powers itself off to a very large degree. Power consumption in this state is very low, your laptop can 'survive' for days in this state. During suspend state your computer more or less stays on, ready to be woken up in an instant. This of course uses more power.

So as for power consumption you could say that there is indeed not much of a difference between power off and hibernation. As a result, having your laptop powered off or hibernated does not require much planning or attention.

The suspend state does however, as your battery will drain much faster. I think the light acts as a reminder to resume whatever you where doing, or to power off your laptop if you won't, saving you from an empty battery.

It also has to do with intention. Quickly closing the lid or putting it to sleep usually means you're going to want to resume using it in the near future. The light acts as a reminder of this intention, telling you to either resume working or power off.

It requires you to act upon that information. And even if you don't power it on - making the decision to let the battery drain - you did know about the state, making the decision an active one.

Most laptops use hybrid sleep (more info) anyway these days. The intention part still holds for this state though.

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"making the decision to let the battery drain" - generally speaking though, you don't. Both Windows and desktop Linux distros somehow get it to fall back on hibernation if the power gets too low during the suspend state. (Practically, sure, maybe the manufacturers think it's a risk so they include it anyway. Theoretically, though, I'm wondering if it's really necessary.) –  detly Oct 21 '11 at 8:20
    
I do like your point about workflow intent, though... I can imagine some sort of, "oh, I had a document open before the phone rang, that's right" use case. –  detly Oct 21 '11 at 8:21
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@detly Indeed, it's a very useful feature. Your battery will be quite low at that point though. My laptop has to be recharged at that point to be useful again. –  TomvB Oct 21 '11 at 10:05
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@TomvB in windows you can also set it to hibernate after a time period rather than a battery %, which can help save your battery if you'll leave it alone for a long period of time. –  Ben Brocka Oct 21 '11 at 14:21
    
@BenBrocka Nice! Didn't know that. I'd assume not everyone does though. –  TomvB Oct 21 '11 at 14:32
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I'm not sure, but I think it is analogous to a pulse, meaning that it is still "alive" in the suspended state and drawing power.

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Yes, and it could potentially wake up from very little provocation, like in your backpack and then if it's not quick enough to handle overheating it might possibly melt or take damage? At least that's how I read the pulsating light - beware of the sleeping beast, he/she's easy to wake up - for better or for worse alike! ^^ –  Oskar Duveborn Oct 26 '11 at 18:28
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