A large file called "sleepimage" under /private/var/vm/

Additionally, if you look at your console messages, whenever you put your comp to sleep, you'll get something similar to this:

30/07/08 6:29:38 PM kernel hibernate image path: /var/vm/sleepimage

That (I believe) is the file you're talking about.

As has already been said, it saves the entire contents of your RAM to the hard drive so that if the power goes out you don't lose your work. You can change your default sleep settings so that safe sleep is off using Terminal, but it's generally not a good idea. It will prevent that sleep image from being created so you can delete it and recover some space on your hard drive, but if your laptop loses power during sleep nothing will be saved and it will just shut off (like yanking the power cord out on a desktop).

If you're pretty confident you're never going to let your battery get to down to 0% and would rather have that extra space, open Terminal and type "sudo pmset -a hibernatemode 0" (without the quotes) and hit enter. Then you can delete the file and empty the trash. If you want to change it back, it's the same command, but replace 0 with 3.

No Einstein... but a little googling goes a long way!

There is no need to touch it.

The sleepimage is the contents of RAM saved to disk every time your Mac is put to sleep. Thus, if you battery gets critically low, you Mac can go into hibernate mode (turns off all power to RAM). When you restart (hopefully attached to a power cord), then it reads the sleepimage to restore you to the exact point you were before the computer went to sleep.

Cheers,

Rodney

use this [link|http://www.macworld.com/article/53471/2006/10/sleepmode.html].