Charles Moore's Mailbag

The 'Safe Sleep' Mailbag

Charles Moore - 2009.06.15 - Tip Jar

In the previous Miscellaneous Ramblings Mailbag, I posted a query from a reader named Richard asking why his MacBook takes a longer time for the hard drive to settle down after he selects sleep mode than older PowerPC 'Books did.

I unfortunately wasn't able to answer that, but as we shall see below, a veritable cast of thousands rose to the occasion.

Ironically, just a few hours after I filed the Mailbag with my reply to Richard on this topic, I happened to run across an article on RecoveryForce discussing that very issue and cluing me in as to why the sleep lag.

Actually, I then recalled that had heard about "Safe Sleep" some time ago when it was announced, long before I bought my MacBook, but it had slipped my mind.

Thanks to everyone who who wrote with explanations and links.


Safe Sleep Writes to Disk

From Dan Knight:

Charles, please refer Rich to this article: How to Disable Safe Sleep in Mac OS 10

What's happening is that his MacBook is writing the contents of RAM to the hard drive before it truly goes to sleep. This makes restarts faster, but it does risk the drive if the 'Book is moved before backup of RAM is completed. The article explains how to disable this behavior.


MacBook Sleep - Why It Takes Longer

From Josh:


It is my understanding that MacBooks take 20-30 seconds to go to sleep because the computer is storing all of the contents of the RAM onto the hard drive. That way, if the computer stays asleep so long that the battery runs out, when you plug the computer back in, it will simply restore itself to the same state it was in when you triggered sleep in the first place - all the same applications, windows, even webpages come back to the way they were. It's pretty cool, actually - similar to hibernate mode in Windows, but much slicker, as is generally the case with Apple products.

Incidentally, Apple recommends that MacBook owners refresh their batteries monthly by letting them run all the way down until sleep is triggered, hard powering down, then powering back up again. When you do this, however, let the computer go fully to sleep before powering down - otherwise, you're cutting off power while the hard drive is still spinning - never a good idea.

Best regards,

Re: Unexpected MacBook Sleep Behavior

From Patrick:

Hi Charles,

I noticed your recent exchange on Low End Mac with another reader about newer Mac portables and the amount of time they take to complete going into sleep mode. I encountered this issue back when I got my current work machine (then brand new) in early 2007. On investigation I learned that Intel Macs write the entire contents of RAM to their hard disks on entering sleep by default. The process is called Safe Sleep apparently, and PowerPC portables didn't use it by default. In the case of my machine, this meant rewriting a 3 gig file to disk every time I closed the lid, not a lightning fast operation and (to my mind) an excessive safeguard.

I discovered that it's possible to reconfigure newer Macs to use the older sleep mode (where RAM stays powered but isn't duplicated to disk). The terminal commands are simple, and as well as speeding up the sleep process and reducing disk wear, it allows the user to reclaim a substantial chunk of storage space. There's an excellent MacWorld article summarising the issue and outlining the various options for changing it: Set Newer Portable Macs' Sleep Mode.

I've now been using my MBP in old fashioned "unsafe" sleep mode for two years and haven't yet had cause to regret changing it. I hope this is helpful to you and your readers.

- Patrick

Hi Patrick. It is. Thanks for the advice and link.


Unexpected MacBook Sleep Behavior

From Sam:


In your May 27th column, Rich asked about the apparently odd sleep function in his Intel Mac. It's actually a feature called Safe Sleep. When you shut the lid, your entire RAM state is saved to disk. This ensures that in a power failure, you can resume working where you left off. If you, right now, close the lid of your MacBook, wait for the sleep light to begin pulsating, and disconnect all power sources (battery/power adapter), the light will go dark. When you press the power button, you will see an odd grey screen with a progress bar at the bottom center. When it completes, you will be shown whatever you have the MacBook set to do upon waking from sleep.

This allows for quick swapping of batteries and replaced the reserve power found in PowerPC 'Books. It can be turned off with the following terminal commands:

  • Turn Safe Sleep Off: sudo pmset -a hibernatemode 0 Delete Sleepimage
  • Saves ~size of RAM on hard disk: sudo rm /var/vm/sleepimage
  • Turn Safe Sleep back on: sudo pmset -a hibernatemode 3

Safe Sleep may be a hassle for you if you rarely run your MacBook unplugged or away from power. Otherwise, you should leave it on, being cautious not to move the 'Book until the noise has ceased.

- Sam

Thanks for the info, Sam.


MacBook Sleep

From Chris:

When putting a MacBook to sleep, the hard drive spins for about 20 seconds as it is saving memory onto disk. You can take the battery out and pull the power cord after that, and it will still wake up in the state that it was in before you put it to sleep, which is pretty clever if you ask me.

This situation is perfect for someone who put their MacBook to sleep a month ago running on battery power and still don't lose anything from the state that it was last in despite the fact that the battery ran out of juice 2 weeks beforehand

Greetings from Chris

Safe Sleep

From Henry:

Hi Charles,

Your new MacBook saves the contents of RAM to the hard disk before going to sleep in case the battery gets low. PowerBooks didn't; they kept RAM refreshed, as do the new 'Books, on battery power. However, if the system sleeps long enough to run down the battery and shuts down, the PowerBooks will lose RAM contents and have to do a cold boot. The MacBook can reload RAM from disk and resume where it went to sleep.

You are right to wait for the sleep light before moving your MacBook, since the disk is in use.


Re: MacBook Sleep Behaviour

From Richard:

Hi Charles,

Regarding the mailbag entry about the MacBook taking a long time before the power LED starts pulsing - I believe this is due to the "Safe Sleep" feature on the Intel Macs. As well as keeping the RAM powered to preserve memory contents, Intel Macs and late-model PowerBooks also save the contents of the RAM to disk when they go into sleep mode. This means that in event of the battery being removed or going flat, the memory contents are preserved on disk, and will be copied back from disk to RAM when the system is powered on. The downside of this is, of course, that it takes longer to go into sleep mode as it copies the RAM contents to disk.

This behaviour can be controled with the Deep Sleep dashboard widget ( ) that allows Safe Sleep to be disabled, permitting a faster sleep by not copying the RAM to disk. It also allows you to force "Deep Sleep" mode, where the RAM is copied to disk and then the system is shut down, emulating the "Hibernate" function of Wintel machines. This mode uses no power at all while sleeping, but takes longer to wake up.

Hope this helps

Richard Halkyard

Hi Richard,

That Dashboard widget sounds like a low hassle way to control this behavior.

Thanks for the link.


Re: Unexpected MacBook Sleep Behavior

From Kev:

I was reading about your and Rich's experience in the last Miscellaneous Ramblings mailbag (05/27). I think the lag before sleep ensues is the "safe sleep" feature kicking in. Simply put, it copies the contents of the RAM to the hard drive, similar to a PC's hibernation, in the event that your MacBook looses power. This allows you to swap batteries while keeping your work going. You might try it out to see how it works.

Kev Kitchens

Odd MacBook Sleep

From John:

Hi Charles,

Apple introduced Safe Sleep sometime in the PowerBook G4 era after I bought mine in early 2003. Older 'Books rely entirely on their battery powered memory contents, which will drain within a few days and force a reboot when the Mac is next turned on.

Newer models save the entire contents of their memory - often 1-4 gigabytes now - to their hard drive in the moments after you close the lid to send them to sleep. That way, all battery can be lost yet the session will be safe to restore from the hard drive. It's a combination of sleep and hibernation. Typically, you open your Mac back up again while the battery is still good, so you don't trigger the full image load from the hard drive. But for safety, the image is written every time the system enters sleep, which is likely what's been noticed.

It's possible to alter the behaviour via some Terminal magic. But the defaults are quite safe and best left be.

- John

Regarding 'Unexpected MacBook Sleep Behavior'

From Sylvain:

Hi Charles,

Here are just a few thoughts about Rich's question and your answer on how his new MacBook has a different sleep behavior than his iBook G4. Indeed, newer Apple laptops take a few seconds before entering sleep, while older laptops sleep almost immediatly. In my understanding, this is implied by what Apple calls "Safe Sleep": Before entering sleep, Mac OS X writes the whole content of the RAM to the disk. This can be found in the file /private/var/vm/sleepimage.

That way, whenever the computer loses all power while asleep (when the battery gets completely empty after a couple of days), you can find your current work, open documents and applications just as you left them when you plug it back in, without having to restart.

This is well explained in that old Mac 911 article at Macworld.

Should you want it, you always can disable Safe Sleep and get rid of the sleepimage file to stick with the old behavior through a few command lines, or with a GUI utility, like for example SmartSleep.

I hope I have well understood Rich's question and this is not old news for you, and I apologize for my bad English, since it's not my main language, and hopefully I haven't misunderstood the whole thing!

Best regards,

Hi Sylvain,

Your English is just fine!


Safe Sleep

From David:



It is called Safe Sleep. That sleep delay is the contents of RAM being written to disk as a file named "sleepimage". The size of the file is the same size as the total RAM.

From Apple: About Safe Sleep

To revert back to the old way requires a trip to the Terminal, as nicely detailed in this Macworld article.

I had set aside a smaller partition to boot in for general maintenance. With 4 GB of RAM, the sleepimage file was too big. The article in Macworld was quite helpful in recovering the free space. I felt that having the RAM backup file in this kind of partition was unnecessary.

I have seen the RAM recovery from disk in action. The display content becomes ghostly white with faint images and a progress bar. After loading is done, it all goes back to normal.


Hi David,

Good point about the issue with small hard drive partitions.


MacBook Sleep vs. PPC Sleep

From John:

Intel-based MacBooks all do something different called Safe Dleep. It can take up to 15 seconds for the sleep light to begin pulsing. Prior to that, the hard drive is still spinning. It is basically paging out the contents of memory to the hard drive.

Wake from sleep is faster.

I was used to my old iBook's instant sleep by closing.

Highly recommend you do not begin moving Intel Macs around until the sleep light is pulsing. Hard drive could get hosed or you could (through jostling the sleep switch) cause a between sleep/wake crash/hang which cannot reach system logs and requires force power off and causes it to heat up a lot in your bag.


'Slow Sleep' Behavior

From Rich:

Charles (and Dan):

Thanks for the article about the 'slow sleep' behavior of my MacBook. The article referenced had a macro for altering the MB's sleep process, but I've since found a Preference Pane that will do the same thing:

Apple also offers this through their Downloads page.

Thanks guys!


Hi Rich,

Thank you for the info and links!


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Charles Moore has been a freelance journalist since 1987 and began writing for Mac websites in May 1998. His The Road Warrior column was a regular feature on MacOpinion, he is news editor at and a columnist at If you find his articles helpful, please consider making a donation to his tip jar.

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