Screen goes dark, then off

Hi
I have my white iMac, and it is in the current phase of the screen going slightly dark, as if I left it for a while, but it now does it whilst I am using it, so during an essay it will go dark. This is annoying as it says that it is on full brightness, and also that I havent put any settings for it to do that. Then, after another 10/20 mins, the screen turns off, but the computer is still on. I can hear the volume clicks as I press the buttons, and I can play music from iTunes ect, but the screen is black. I have to restart and I lose all my stuff I was working on, but the problem is gone for another few days. I have checked the power and it isnt a problem with that.

Any ideas?

Are you using a bluetooth keyboard?

If not it sounds like a sleep issue - if it is still under warranty take to to Apple

there is a discussion about the problem here.

http://discussions.apple.com/thread.jspa?threadID=1158374&tstart=75

Nope Regular keybord