Different types of sleep, no rhyme or reason

[SteelCity]

I have my iMac set to sleep after 15 minutes, both monitor and computer. I also have it set to "Put hard drives to sleep when possible".

Sometimes when I come back to my iMac I could bump the mouse and it will immediately turn back on. Other times bumping the mouse does nothing so I hit Enter and the computer makes a noise (I believe it's checking the CD-ROM) and then wakes up after 3-4 seconds.

I can't find the reason why it does two different things. I only really run Safari and when I leave the computer I minimize it, that's the way I always leave the computer when I leave it. Sometimes I'll come back after 30 minutes and have to wake it up with the keyboard. Other times I'll come back after 12 hours and just bumping the mouse will turn the screen back on.

Anyone know what's going on?

Thanks!

 

[Makosuke]

Actually, only one of those behaviors is a full system sleep. The "lighter" of the two is just the monitor turned off automatically--the computer is still awake and running (if it had louder fans, you'd be able to hear them).

The monitor should sleep so long as there isn't a video actively playing, but the computer will only go into a full sleep if the OS decides that there's nothing significant going on. As for what particular things qualify as "significant," it's hard to say, but obviously something that you leave running regularly is registering activity that prevents the computer from sleeping.

Mail's auto-check used to do this, but I think that's been fixed now. My top guess would be something Flash-based running in an open browser window--it's entirely possible that counts as activity. The easy way to test would be to either experiment with closing open browser windows before walking away from the computer and see if there's a relationship, or just look at everything that's open after returning to a "not really sleeping" system--if you can find a Flash-based banner ad (VERY common) or some other Flash-based doodad, then that's probably it. If there is definitely none, must be something else.

There is also hardware that can cause this, so another thing to try would be to see if you always have, say, a particular pen drive connected when it doesn't sleep.

Of course, the easiest solution is to just tap the power button when you get up--if the correct box is checked in the Options tab of Energy Saver, that will immediately sleep the computer no matter what. So long as there isn't a download or some other long operation in progress (which would get cut off), that guarantees full sleep, and does so immediately. It's what I do, anyway.

 

[SteelCity]

Thanks for the reply!

One more question, what benefit does turning off the hard drive give me? Should I use that option?

 

[Makosuke]

One more question, what benefit does turning off the hard drive give me? Should I use that option?

Pretty much what it sounds like--some power savings while the hard drive isn't spinning, offset by a few-second delay when the computer tries to access it and it needs to spin up. The tradeoff is more important for a laptop where extra juice is at a premium, though in general it'll depend on your use pattern.

You can find people who will argue that the more frequent spin-up/down cycles will shorten the life of the drive, but I'm somewhat skeptical of this, and I've seen no manufacturer statements backing this up. Could be true, but it's certainly not going to drastically change the lifespan of a desktop-grade drive either way.

Personally I have the feature on on my 24x7 home server, since it's not doing anything with the drives during the majority of the day so they might as well be off, while on speed-critical systems I disable it.