Its because when you are tired, your muscles start slowing down and getting "tired" too. The weakest muscles are the first to show signs of this - which are in your eyes. That is why you get the "heavy lids" from being tired, and rubbing them is an unconscious way to stimulate the eye muscles and get them to work better. That's also why you have bags under your eyes when you don't get enough sleep - sleep regenerates muscle tissue, which gets tiny tears and wears out during the day from use, resulting in blood vessel breakage. So when you don't get enough sleep, those muscles don't get repaired, and thus the "eye bags" appear.
at 8:30 PM on Feb. 6, 2010