Citizens of the US are putting on weight as a result of eating more, though it may not be as simple as this lone factor. Barry Popkin, a professor of nutrition at the University of North Carolina, Chapel Hill, discovered in his research paper for the American Council on Science and Health that people in the States are now eating more than they did in the 1970s. Back between 1977 and 1978, an average of 1,800 calories was consumed, yet now it is almost 2,400 (2003 to 2006). However, it was mooted that it may be more than extra calories that is making people fatter; Dr Popkin said a lack of good-quality sleep may also be a major contributor. Americans now average 6.5 to seven hours of sleep a night, compared to 8.5 hours they got in 1960. One study said the average level of leptin, the hormone that controls feelings of fullness, fell by 18 per cent when sleep dropped to four hours per night over two nights. Meanwhile, a hormone that triggers hunger - ghrelin - increased by 28 per cent. Dr Josh Bloom, who worked on the research, said: "It could be as simple as being awake more hours gives you more time to eat."