The clock in your brain doesn't just take cues from light, but from the hormone melatonin as well. Every night, about two to three hours before you conk out, your brain starts to secrete melatonin in response to darkness. Taking a melatonin supplement in the evening will advance your internal clock and make it possible to fall asleep earlier; taking it in the morning will do the opposite. (You might assume this would make you even more tired during the day but it won't; you could think of it as tricking your brain into believing you slept longer.)
So now you know the fix for jet lag: Travel east and you'll need morning light and evening melatonin; go west and you'll need evening light and morning melatonin.
The same principle tells us what to do for night owls, the 5 percent to 10 percent of adults who don't start releasing melatonin until late. If they try to sleep at a normal hour, like 11 or midnight, they will have "insomnia," because they don't feel sleepy yet — their natural circadian rhythm is delayed.