"Nodding off" isn't just a figure of speech - there is a real phenomenon of briefly falling asleep against your will. Sleep medicine has a term for this: microsleep.

When you have a microsleep, you go to sleep for a half second or longer, up to thirty seconds. You don't notice it because it’s just like falling asleep at night - unconsciousness hits you before you see it coming. If you're watching TV the story will seem disjointed and hard to follow. If you're driving you may find yourself veering down the side of the road all of a sudden.

Microsleeps can impact everyday life in subtle ways. Even the briefest microsleep of a split second will disrupt your ability to follow a conversation, stay focused on a task, or learn something new. These small moments, added up over time, become very discouraging. You lose confidence in yourself and a wall seems to form between you and others. Being sleepy isn’t just going about things more slowly; your brain is rebooting itself every few moments. Sleep deprivation makes you less present in a very real way.

Take a look at these articles on microsleep:

http://www.sleepdex.org/microsleep.htm
http://www.sciencedaily.com/releases/2011/04/110427131814.htm