AF systems find it extremely hard to function without light. Need to focus in the dark? here is a quick simple trick by Dan Reinecke (eHow). Since the AF can’t function in the dark under normal conditions, increasing the sensitivity (camera ISO) to the maximum should allow the camera to focus in live view in most situations.
After you finished focusing move your camera/lens into manual focus mode. Be sure the camera won’t move (preferably using a tripod) and of course don’t forget to change your sensitivity to lower settings so you won’t get a grainy image.
You can watch many more photography tips on the LensVid technique section.