If you want to shut your sleep timer off for good, eHow has a pretty great round-up of how to navigate your computer system's settings to do it.
But if you need some temporary relief from the impending dim of sleep mode, try one of these easy-to-use (and free!) solutions:
- Caffeine. It's a free program for your menu bar, available through the Mac App Store. Once it's installed, you'll find a coffee cup icon in your menu bar. All you have to do to temporarily disable sleep is get a hit of digital espresso by clicking the coffee icon. Your computer won't sleep until you click the icon again. You can also use the Caffeine menu to prevent your Mac from sleeping for a preset length of time. [Mac App Store]
- Insomnia. A free application for Windows PCs that tackles the same task as Caffeine for Mac. Once you open Insomnia, this app will open a small window on your desktop that keeps your computer from sleep. To go back to allowing power-saving sleep, just close the window. The Insomnia window sits on top of whatever you're doing, an annoying quirk that'll ensure you remember to shut it down and let your PC get some rest. [MSDN Blog Download]