You can feel your back door puckering in anticipation.
Logic dictates that you're probably just releasing head pee as a sort of relaxational process your body likes to go through as you wind down at night, because your body is a wonderland of gross when you're not paying attention to it. That's the scientific explanation for farts, incidentally -- your body attempting to be funny. Usually it works. Farts are hilarious. Or maybe it's just because your hair is naturally a bit greasy and if you hold something against it for eight hours a day, every day, it's going to get greasy, too. Feel free to test this theory by taping a swatch of cotton to Ron Jeremy's back for part of the day and see what it looks like when you're done. Just revolting.
If you Google "yellow stains on my pillow" right now, you'll get thousands of results from poor, confused, greasy Yahoo users desperate to understand why their hair follicles insist on jacking it all over their pillows at night with only the educated guesses of other Yahoo users to satiate their curiosity. Unfortunately, the people who provide info on Yahoo Answers generally only have Ph.D.s in meth production and dumbassery, so their answers are about as satisfying as mine, but with more typos and a general air of discontent over their inability to understand the metric system and how you can tell if a door is a push or a pull without a sign. As near as I can tell, no actual scientific body has bothered to write about this phenomenon, because no one important actually cares. It's really disturbing to the good people of eHow, however.