Scientists from Stockholm's Karolinska Institute took two sets of photos of 23 men and women: Once after the subjects had eight hours of sleep, and once after they'd spent 31 continuous hours awake. (For the photos, the subjects maintained the same facial expression, the same distance from the camera, and didn't wear make up either time.) A group of observers then rated each photo based on a series of qualities, including health and attractiveness.
And! Surprise, surprise, people who have been up for 31 hours in a row look a little bit, uh, off: "The observers rated the sleep-deprived as less healthy, more tired and less attractive than those who had had enough sleep." So, yes, the fact that you stay up till 2 a.m. wondering if the convenience store clerk was mad at you for asking for change, that is the reason you are ugly.