Why won't my computer sleep?

The computer runs fine, but upon closing the top it momentarily sleeps, but you then hear the hard drive spin and the apple lights up. I have tried manually putting it to sleep, but have the same problem. If I leave the lid open to let it put itself to sleep, the screen saver eventually freezes, and the screen remains lit until I wake it. Any suggestions would be greatly appreciated. I am thinking about replacing the logic board, but I am unsure as to whether the problem is related to that.

Thanks, Charl