I've never understood why Apple conceals useful operating system features from its users. I'm talking about such things as changing the default file format for creating a screen shot, or turning off the Dashboard app.
Sooner or later (usually sooner) some clever Mac geek figures out how to access and enable these hidden options via the Unix command line in the Terminal or writes a utility to facilitate the task.
Such is the case with Deep Sleep, a Dashboard widget that lets users put their Mac into a standby mode that completely shuts off the power, similar to the "hibernate" function in Windows.
I stumbled upon Deep Sleep in my continuing quest to resolve my Mac Pro's reboot-on-wake problem. Several Mac Pro owners in the forums had suggested use of this widget to bypass the issue.
It turns out that the default Sleep mode on a Mac is only one of three Sleep options in the operating system. The others are Safe Sleep and Deep Sleep.
In the default Sleep mode, the Mac maintains a continuous trickle of power to the computer's memory. Without that stream of electricity, everything stored in the Mac's memory (that is, whatever you have running on your Desktop) would go poof in a nanosecond.
Because the information is preserved in memory, wiggling your mouse or tapping a key on the keyboard brings up your Mac Desktop just as you left it.
Apple created the Safe Sleep mode for PowerBooks a few years ago, gradually making it a standard feature on all Mac laptops. To prevent data loss, Safe Sleep adds an extra step. It saves the contents of memory to the hard drive just before going into regular Sleep.
With regular Sleep, if the PowerBook's battery ran out of juice, the computer's memory would lose its lifeline of electricity. The state of the Desktop would be lost, and any unsaved work in open Documents with it. But Safe Sleep could retrieve that data from the hard drive. Once power to the laptop is restored, so is the state of the Desktop.
The Deep Sleep widget uses the Safe Sleep option of copying the contents of memory to the hard drive to eliminate the need for that memory-preserving trickle of electricity in desktop Macs as well as laptops.
The advantages of Deep Sleep over regular Sleep is that 1) it saves your open files to the disk so you won't lose any data, and 2) it uses virtually no power. For laptop owners, the ability to sleep the Mac without consuming any battery power can be invaluable.
The primary disadvantage of Deep Sleep is that it takes several seconds longer to put the system to sleep (because the Mac must wait for the contents of memory to write to the hard drive) as well as to wake it up (because the Mac has to read the data off the hard drive).
I have been using Deep Sleep when I plan to leave my Mac Pro unattended for many hours (such as when I leave for work or go to bed at night) and use regular Sleep the rest of the time. I started this to work around my reboot on wake problem, but I think Deep Sleep might be better for the Mac.
As for the potential power savings, don't get too excited. According to an energy usage calculator on Apple's Web site, a Mac Pro running 24/7 (never sleeping) would use $120 in electricity in a year at 10 cents per kilowatt-hour, slightly less than the 10.86 cents Baltimore Gas and Electric Co. currently charges. (A 24-inch iMac would cost $123 because of the built-in monitor; a 20-inch iMac $74.)
Using regular Sleep mode 12 hours a day would save a Mac Pro owner $57. Using Deep Sleep 12 hours a day should save you $60 - just $3 more (assuming the Mac is drawing no electricity).
Not quite enough for a new iPod, but it could get you a movie rental from the iTunes Store.