Up close with Mountain Lion: Power Nap

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When your Mac is connected to AC power, Power Nap performs all of the above tasks, as well as tasks that require a bit more power, longer wake times, and extended data transfers:

  • Backs up using Time Machine if your Time Machine drive is connected (hourly until a successful backup has been completed).
  • Checks for--and downloads, but doesn't install--new updates to Apple software and OS X (once per day).
  • Checks for updates to Mac App Store-purchased software (once per week). If updates are available, you see an onscreen notification with Details and Update buttons. Clicking Details opens the Mac App Store and displays the Updates screen--with available updates already listed, rather than making you wait for an update check. Clicking Update instead installs those updates immediately. (Power Nap won't automatically download app updates, but if you've started to download Mac App Store updates and then put your Mac to sleep, Power Nap will attempt to finish the download[s] during scheduled wakes.)
  • Performs Spotlight indexing, if necessary.
  • Updates OS X's Help Center data, if necessary.

I've also noticed that since I enabled Power Nap, my MacBook Air wakes up instantly. It used to take a few seconds for the computer to be fully responsive, but that delay is gone. I don't know if this is directly attributable to Power Nap having already handled some of the tasks that would normally occur on wake, but it's a welcome improvement.

How do I know if Power Nap is working?

Given that Power Nap does its thing while your Mac is asleep, and flash-storage Macs were already very good about waking up quickly and syncing many types of data silently in the background, you may be wondering if Power Nap is actually doing anything.

If you've got software updates waiting, the aforementioned notification is the most-obvious confirmation. But if you go for a week or more without any of your software needing an update, you won't see that notification. You could quickly switch to Mail to see if there are any new messages waiting, but Mail also checks for new Mail as soon as you wake up your Mac, so even that may not be a foolproof approach.

One easy way to confirm that Power Nap is doing its thing is to check Time Machine. Click Time Machine's systemwide menu, or open the Time Machine pane of System Preferences, and then look at the Latest Backup time. If it happened when your Mac was supposedly asleep, Power Nap is working.

If you don't use Time Machine, or if your Time Machine drive wasn't connected, another approach is to check your Mac's logs. Open the Console app (in /Applications/Utilities) and, in the list on the left, select All Messages. If Power Nap is working, you should see entries with times and dates that fall during the time your Mac was asleep. For example, here are a few lines from a 2012 MacBook Air's logs (I've deleted several lines to shorten the list):

2:11 AM, 7/31/12 2:11:57.000 AM kernel[0]: AppleUSBMultitouchDriver::checkStatus - received Status Packet, Payload 2: device was reinitialized

2:11 AM, 7/31/12 2:11:58.000 AM kernel[0]: Graphics suppressed 1238 ms

2:11 AM, 7/31/12 2:11:59.000 AM kernel[0]: MacAuthEvent en0 Auth result for: 00:25:bc:89:d1:0a MAC AUTH succeeded

2:11 AM, 7/31/12 2:11:59.000 AM kernel[0]: AirPort: Link Up on en0

2:11 AM, 7/31/12 2:11:59.957 AM awacsd[74]: InnerStore GetWakeInfoForZone: zero address for 38250235.members.btmm.icloud.com.

2:12 AM, 7/31/12 2:12:28.281 AM com.apple.backupd-helper[4308]: Not starting scheduled Time Machine backup - time machine destination not resolvable.

2:12 AM, 7/31/12 2:12:48.296 AM com.apple.time[11]: Next maintenance wake [Backup Interval]: [date: 0x7fca9be0c640] Tue Jul 31 03:12:28 2012 PDT (approx)

2:12 AM, 7/31/12 2:12:48.296 AM com.apple.time[11]: Requesting maintenance wake [Backup Interval]: [date: 0x7fca9be0c640] Tue Jul 31 03:12:28 2012 PDT (approx)

2:12 AM, 7/31/12 2:12:50.000 AM kernel[0]: AirPort_Brcm43xx::powerChange: System Sleep

As you can see, the Mac woke up with graphics disabled, connected to WiFi, tried to back up using Time Machine, scheduled the next wake, and then went to sleep. (Not shown for space reasons are log events for email checks and calendar updates, as well as a bunch of more-technical stuff.) If you scroll through your log, you'll see a similar group of events roughly every hour.

That said, we've received a number of reports from readers that Power Nap doesn't seem to be successfully performing all of its tasks. For example, some people report that their Mac isn't retrieving new email messages during sleep. We're looking into this, and we'll update this article as we get more information.

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