You probably still have the default settings for the backlight on. You can change how long/when sleep mode occurs in the System Preferences.
1. Go to "System Preferences" and click "Energy Saver."
2. You can change how long your Mac must be inactive before entering sleep mode. You can change settings for if it's connected to the AC or if it's running on battery.
3. If you don't ever want your Mac to enter sleep mode without you telling it to, you can download InsomniaX. This program keeps your Mac awake even if you shut the lid. If you want your Mac to sleep, you just click the little blue dude in your menu bar and disable it. Download InsomniaX here: http://www.macupdate.com/info.php/id/222...
Hope this helps! :)