What happens when you don't have enough sleep?

No one knows for sure why sleep is necessary, but there's no doubt that getting too little throws a wrench into your body's works. For example, studies show that a sleep debt lowers levels of the hormone leptin, which helps keep your appetite under control. Implication: Sleep too little, and there's a good chance you'll be soon overeating. Sleep deprivation also boosts levels of stress hormones, which prompt your body to send more glucose into your bloodstream. Too little sleep also makes your body less sensitive to insulin.

But that's just the beginning. Research shows that sleeping too little shuts down production of certain chemicals in the immune system that defend your body against germs. Shortchange yourself on shut-eye and you may want to have a box of tissues and cough medicine handy: A 2009 study found that people who sleep less than seven hours a night are up to three times more likely to develop a cold.

Other studies show that even modest sleep deprivation - cutting back from your usual eight hours a night to six hours, for instance - can turn up levels of chronic inflammation, which increases the risk for many conditions, including heart attacks, strokes, and osteoporosis.

Then, there are the immediate effects. When the alarm clock blares you out of a deep sleep, you're apt to start the day in a sour mood. As the day passes, you're also likely to feel dull witted and foggy. Some neurologists believe that one purpose of sleep is to give your brain a chance to build and strengthen the wiring between neurons. Studies show that well-rested people learn new information faster and have sharper memories. Short sleep reduces your reaction time, too, making you at risk for car accidents and other mishaps.

While some people can get by on relatively little sleep, most of us need seven to eight hours a night. Experts say one sign that you're getting adequate sleep is that you can wake up on time every day without using an alarm clock.