Unchecking this option in System Preferences > Energy Saver will prevent your MacBook’s screen from dimming before it goes to sleep.

Whenever I’m watching a video on my MacBook, the screen dims after a few minutes, and I have to move the mouse to get my screen back to full brightness again. I hate this! The screen’s not going to sleep; it’s just dimming a little bit. How can I prevent this?

Go to System Preferences > Energy Saver and click on the Options tab. There you’ll see a checkbox labeled “Automatically reduce the brightness of the display before display sleep.” Uncheck this box and your display will no longer dim until it’s time for your display to sleep, at which point it will go completely black. Also note that if you’re watching a full-screen video in QuickTime Player, or if you’re watching a DVD with DVD Player, your display will neither dim nor sleep regardless of this Energy Saver setting.

If you’ve already made this change in System Prefs and your display is still dimming before it sleeps, you may be experiencing a known issue with Intel Macs running Tiger (Mac OS 10.4). See Apple’s tech info article at docs.info.apple.com/article.html?artnum=304092 for some tips on working around this issue.