Enlarge Image Self-medicated. A new study suggests that some people suffer from excessive sleepiness due to a naturally occurring compound that acts like a sedative. Credit: Digital Vision/Thinkstock Hitting the wall in the middle of a busy work day is nothing unusual, and a caffeine jolt is all it takes to snap most of us back into action. But people with certain sleep disorders battle a powerful urge to doze throughout the day, even after sleeping 10 hours or more at night. For them, caffeine doesn't touch the problem, and more potent prescription stimulants aren't much better. Now, a...