This is the VOA Special English Health Report.

The next time you think about going without sleep, consider this: Laboratory animals that are kept awake for long periods of time ... die.

Yet sleep scientist Ying-Hui Fu at the University of California, San Francisco, says little is known about the basic need for sleep.

YING-HUI FU: "We do not know why some humans need more sleep; why some humans need less sleep; why, when we do not sleep, we do not function well. We just do not know much about sleep at all."

But here is something that scientists now know: A team led by Professor Fu has reported the first genetic link to how much sleep we need.   

The team was looking for a natural clock in the body that controls sleep and wakefulness. What they found was a genetic abnormality. People who have this mutation need less sleep than others.

But keep in mind that the scientists say this mutated gene may be rare. The study involved two members of an extended family. They did fine on just six hours of sleep a day. Studies have shown that over time, most humans need eight to eight and a half hours of sleep for the best health.

To test their theories, the scientists genetically engineered the mutation in mice. The mice with the mutation needed less sleep than normal mice. They were also more active even after being kept awake.

The study appears in the journal Science. The researchers will continue to study the mice to test whether the gene is related to other medical conditions. And they will study whether it is involved in controlling sleep quantity alone, or also what scientists call the "wakefulness-behavioral drive." This drive is important for getting food, shelter and mates.

How you sleep can be as important as how much you sleep -- especially for newborn babies. A new report says images in parenting and women's magazines may send the wrong message about how to put babies to bed.

The study found that more than one-third of the pictures in women's magazines showed babies in unsafe sleep positions. They showed babies sleeping on their sides or stomachs. Also, only a third of the pictures showed sleep environments considered safe by the American Academy of Pediatrics.

The academy says babies should sleep on their backs. It says they should be placed on a separate sleep surface from their parents, without blankets, pillows or other soft bedding. These guidelines are credited with reducing cases of sudden infant death syndrome in the United States.

And that's the VOA Special English Health Report, written by Caty Weaver with Jessica Berman. For more health news, go to voaspecialenglish.com. I'm Mario Ritter.