There are new findings that not enough sleep may cause people to gain weight.Researchers say a lack of sleep can produce hormonal changes that increase feelings of hunger. In one study,researchers in the United States examined information on more than 1000 people.The people had taken part in a long-term study of sleep disorders. Some people slept less than five hours a night.They had 15 percent higher blood levels of a hormone called ghrelin than people who slept eight hours.And they had 15 percent less of the hormone leptin.Experts say ghrelin helps make people feel hungry;leptin makes you feel full. The scientists say these hormonal changes may be a cause of obesity in Western societies.They note the combination that limitation is common and is widely available. The results were not affected by how much people exercised.People who are awake longer have more time to burn energy.But the researchers say loss of sleep may increase hunger especially  for high-calorie foods,so people gain weight.It seems that,for survival,the body may be designed to store more fat during times with less sleep. Researchers from Stanford University in California and the University of Wisconsin did the study.They found that the best amount of sleep for weight control is 7.7 hours a night. The Public Library of Science published the findings in its journal Medicine.Internet users can read the full study,free of charge,at plos.org. The writer wrote this passage to  _  .
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