Your Mac and Sleep Mode
From Mac Guides
Macs are equipped with a little extra feature that really can be a real time saver, whether you're on the road or just at home working and you need to leave it for a couple of minutes:
The sleep mode.
Whenever you close the lid on your portable mac or press the on/off button once on your desktop mac, the mac will go to sleep. The advantages of this feature opposed to shutting down the mac is that it takes less than 10 seconds to send the mac to sleep and wake it again, opposed to shutting it down and booting up again, which will take approximately 1 minute depending on your mac.
There are several instances of the sleep mode, invisible to the user but let's go through them nevertheless:
Normal sleep mode: The Mac's RAM  will be saved and powered (either through battery on a portable mac or through normal power on a desktop mac) and everything else on the machine will be powered off.
Safe sleep mode: Since the move from PowerPC to Intel processors, all macs use safe sleep as the default sleep mode. As in sleep mode, RAM will be powered but additionally the RAM will be written to the Hard Drive in one page file as a backup if the battery runs out or a power failure happens. This means that your data will be saved no matter what happens, because in the unlucky event that a power failure happens, your mac will simply boot from the page file, loading that file into the RAM and you can continue where you left off.
Suspend mode: The Suspend mode is identical to Windows' Hibernation mode. In Suspend mode the mac's RAM will be written to the Hard Drive and the mac will be completely powered off. This means that when you turn it on again the mac will boot up from the page file, equivalent to if there was a power failure during Safe Sleep mode. The Suspend mode is currently only used on portable macs when the battery is completely drained, ie. when calibrating (. Suspend mode can also be enabled through the Deep Sleep widget  but use it at your own risk, as it is not officially supported by Apple.
Why don't all Macs just use sleep all the time? Good question. Many new switchers from Windows are not used to using sleep (although the sleep mode is somewhat equivalent to Windows' standby function) and are often in doubt how Sleep mode works. Hopefully this wiki article will shed some light on it.
One of the reasons sleep mode is not the preferred way to shut down the Mac is that it doesn't shut it down at all. The Mac will still use battery if portable and still draw power if it's a desktop Mac. This means it's not very energy efficient in the long run if you go a long time without the Mac. Apple currently recommends that you use sleep mode if you need to use the Mac within three days [need a citation here...].
Sleep and maintenance: One of the advantages to sleep is that the Mac will actually run its maintenance scripts which it wouldn't if it was shut down. Mac OS X has three types of scripts: daily, weekly, monthly. If you Mac sleeps and is awakened but missed the time of its weekly script it will execute it immediately. This will not happen if it was shutdown.
Sleep and battery life: When your portable Mac is sleeping, it will consume a little battery life so sleep mode doesn't come without a cost. However, it's very little because only RAM is powered.
Sleep and moving the portable Mac: Since only the RAM is powered and Optical Drive and Hard Drive (the only mechanical hardware with moving bits) is not, you can safely transport your Mac as you see fit.
Sleep and wear and tear: There has been some discussion as to whether sleep mode will wear down your Mac quicker or not. It's not an easy question to answer and it basically runs down to two opposing answers:
- Yes: It will have some effect on your portable Mac since some of the electric circuits will powered and thus used.
- No: Booting the Mac up cold will have more wear and tear overall, since electronic circuits are strained the most when going from an unpowered to a powered state. The amount of read/write access to the Hard Drive when booting contributes wear as well.
There is no definitive answer yet and it will most likely not make a difference.
One final consideration is that sleep mode saves ALL data in RAM, whether it is good or bad. Often times when an application is quit, it will leave things behind in memory. This extra "RAM waste" will be persist and will continue to accumulate from other applications and such until the system is fully rebooted. Computers which run many applications may experience decreased system stability or speed the longer the system goes from its last shut down or restart. Therefore, a normal user may wish to consider fully shutting down or rebooting the system at least once every couple weeks to ensure optimal reliability and operating speed.
Basically, when it comes to sleeping the Mac, you will have to choose for yourself: do you need the convenience in time saved versus battery and power used?