I think you have the wrong idea of what sleeping is. It is when the computer shuts down completely, but saves what it was doing last. Then when you open it up, it goes back to doing what it was doing. When its sleeping, the computer does absolutely nothing. No disk reads, or writes. No internet connection, nothing. This is the same for all computers that have a sleep mode.
Now you can get a hack for the system to leave the Mac on when you close the lid, but that could possibly hurt it. Just leave the screen up, and have the screen turn off.
What hack? Apple supports running it with the lid closed, though perhaps in a different situation that the one to which you refer: when using an external display.