Apple supports three main types of sleep modes for desktops and portables. The three modes are Sleep, Hibernation, and Safe Sleep, and they each work slightly differently.
Sleep: The Mac's RAM is left powered on while it's sleeping. This allows the Mac to wake up very quickly, because there's no need to load anything from the hard drive. This is the default sleep mode for desktop Macs.
Hibernation: In this mode, the contents of RAM are copied to your hard drive before the Mac enters sleep. Once the Mac is sleeping, power is removed from the RAM. When you wake the Mac up, the hard drive must first write the data back to the RAM, so wake time is a bit slower. This is the default sleep mode for portables released before 2005.
Safe Sleep: The RAM contents are copied to the hard drive before the Mac enters sleep, but the RAM remains powered while the Mac is sleeping. Wake time is very fast because the RAM still contains the necessary info. Writing the RAM's contents to the hard drive is a safeguard. Should something happen, such as a battery failure, you can still recover your data.
Since 2005, the default sleep mode for portables has been Safe Sleep, but not all Apple portables are capable of supporting this mode. Apple says that models from 2005 and later directly support Safe Sleep mode; some earlier portables also support Safe Sleep mode.
Find Out Which Sleep Mode Your Mac is Using
You can find out which sleep mode your Mac is using by opening the Terminal application, located at /Applications/Utilities/.
When the Terminal window opens, enter the following at the prompt:
pmset -g | grep hibernatemode
You should see one of the following responses:
Zero means normal sleep and is the default for desktops; 1 means hibernate mode and is the default for older portables; 3 means safe sleep and is the default for portables made after 2005
You can change the sleep mode your Mac is using, but I don't advise it for older (pre-2005) Mac portables. If you try to force an unsupported sleep mode, it may cause the portable to lose data when sleeping. Even worse, you may end up with a portable that will not wake up, in which case, you'll have to remove the battery, then reinstall the battery and operating system. If my portable didn't support Safe Sleep, I'd prefer the reassurance of hibernating over a quicker wakeup from standard sleep mode.
If your Mac isn't a pre-2005 portable, or you want to make the change anyway, the command is:
sudo pmset -a hibernatemode X
Replace X with the number 0, 1, or 3, depending on which sleep mode you wish to use. You will need your administrator password to complete the change.