You should create an account for this purpose, which you can do through the Accounts pref pane.
How dark does the screen get? Is it just a little dim - or does the screen brightness completely shut off (you can see the icons/menus if you look carefully)?
If the brightness is off, you need to contact AppleCare, or take to an authorized service. If it does not go completely dim, have you tried resetting the SMC?
Turn off, and unplug EVERYTHING from the back, including the power cord.
Unplug all cables from the computer, including the power cord.
Wait at least 15 seconds.
Plug the power cord back in, making sure the power button is not being pressed at the time.
Reconnect your keyboard and mouse to the computer.
Press the power button on the back to start up your computer.
If this reset does not change anything, contact Apple.
If you create a different account to try, Logout from your user, and login as the new account. This should have default energy saver prefs, and you can change those from your new account to see if any changes affect your problem.