We have groupings of interacting molecules in cells throughout our body that act as biological clocks, telling our glands when to release hormones and adjusting our body temperature and other variables. Just as the world's watches and bell towers all rely upon the Royal Observatory in Greenwich, UK, the body's tiny biological clocks follow a master timepiece -- 20,000 nerve cells called the suprachiasmatic nucleus, or SCN, located in the brain's hypothalamus region [source: NIH].
The SCN keeps your body operating on a regular pattern of sleep-wake cycles and body functions known as a circadian rhythm. When it's time to get some sleep, for example, the natural time-keeping system releases a hormone called melatonin, which makes us go beddy bye. The SCN knows when it's a good time to do this because it's located conveniently close to the optic nerves, which relay the perception of light from the eyes to the brain. Basically, when there's less light, the SCN tells you to release more melatonin, and you nod off. But the body also likes regularity, so this natural clock gets accustomed to going off at the same time every night [source: NIH].
But your body's timekeeping system has trouble drastically resetting itself that quickly, and when you cross multiple time zones, you get all messed up as a result. The problem is at its worst when you fly eastward, say from Chicago, in the United States, to Paris, France. When it's nighttime at your destination, your body still thinks it's late afternoon. You may wind up lying sleepless in your hotel bed all night, and finally doze off just when it's time to get up for lattes and croissants [source: Eastman and Burgess].
You feel rotten because sleeplessness disrupts just about every biological function in the body. It causes the release of stress hormones, which make you feel anxious and grumpy. It drives up your blood pressure, and sends inflammation-stimulating chemical markers flooding through your arteries. The shift also disrupts the release of appetite-regulating hormones, so that you get a craving to scarf down a lot of food at a time when you normally don't eat at all. And finally, it disrupts your body's regular release of melatonin, which in addition to inducing sleep helps regulate other hormones and protects you against diseases such as cancer [source: Stein].