According to the passage, how many universities in the United States took part in the study?

There are new findings that people with inadequate sleep are likely to gain weight. Researchers say a lack of sleep can produce hormonal changes that increase feelings of hunger.
    In one study, researchers from Stanford University in California and the University of Wisconsin examined information on more than one thousand 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 fifteen percent higher blood levels of a hormone called ghrelin than people who slept eight hours. And they had fifteen percent less of the hormone leptin. Experts say ghrelin helps make people feel hungry; leptin makes you feel full.
    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.
    Researchers found that the best amount of sleep for weight control is seven-point-seven hours a night.
    Researchers at the University of Chicago did a smaller study. They found that people who slept just four hours a night for two nights had an eighteen percent reduction in leptin. And they had a twenty-eight percent increase in ghrelin. The young men in that study also appeared to want more sweet and starchy foods.
    Researchers from Columbia University in New York did a third study. They found that people who got less than four hours of sleep a night were seventy-three percent more likely to be overweight. This was compared to people with seven to nine hours of sleep. The researchers say that for survival, the body may be designed to store more fat during times with less sleep.