Jennifer Matthews: How much sleep is enough?

Female Speaker1: I normally get seven to eight hours a night.

Female Speaker2: Probably five or six hours.

Male Speaker: Nine hours roughly.

Jennifer Matthews: New research shows that question carries more weight than you might think. Lack of sleep causes inattentiveness and makes it difficult to learn, but neuroscientist Jerome Siegel has another question.

Jerome Siegel: The more subtle question is: If you don't get sleep, will it shorten your life span?

Jennifer Matthews: Siegel studies dog with narcolepsy, a serious sleep condition. He's been fascinated with the science of sleep for 30 years. He says sleep studies often uncover more questions than answers.

Jerome Siegel: Under extreme conditions in animals, if they're totally sleep deprived for periods of weeks, they will die. They'll die sooner than if they don't get food.

Jennifer Matthews: During sleep, heart rate, blood pressure and temperature drop. The body also secretes growth hormone, which regulates muscle mass in adults. It secrets leptin too, a hormone that tells the body it's had enough food.

Problems start when you don't enough shuteye. A recent study shows when healthy volunteers were sleep-deprived, they became pre-diabetic after just one week and that's not all.

Esra Tasali: Two nights of sleep restriction to four hours of bedtime resulted in cravings for especially sweets, candies, cookies, types of food that we generally refer as junk food.

Jennifer Matthews: Esra Tasali says sleep loss creates even more problems for dieters.

Esra Tasali: The appetite hormone ghrelin goes up, which again signals the brain that you have to have more food.

Jennifer Matthews: Most scientists agree. Sleep boosts the immune system and Siegel says, it's the only time the brain gets a break.

Jerome Siegel: Unlike muscles, for example, which can rest when you sit down, brain cells are pretty much working 24 hours a day.

Jennifer Matthews: So, how much sleep do you need? One study shows people who slept seven hours a night were more likely to be alive five years later than those who slept six and those who slept eight hours a night.

Jerome Siegel: If you tell people who are sleeping nine hours a night to cut their sleep to seven hours, is there lifespan going to be extended? That seems unlikely, but it's possible.

Jennifer Matthews: Whether you rack up seven hours or not, the important thing to remember is that as you're heading off the dreamland, your body has a full night's work ahead of it. This is Jennifer Matthews reporting.