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Ask anyone why there is an obesity epidemic and they will tell you that its al down to eating too much and burning too few calories. That explanation appeals to common sense and has dominated efforts to get to the root of the obesity epidemic and reverse it/ yet obesity researchers are increasingly dissatisfied with it. Many now believe that something else must have changed in our environment to precipitate（促成） such dramatic rises in obesity over the past 40 years or so. Nobody is saying that the big tworeduced physical activity and increased availability of foodare not important contributors to the epidemic, but they cannot explain it all. Earlier this year a review paper by 20 obesity experts set out the 7 most plausible alternative explanations for the epidemic. Here they are. Not enough sleep It is widely believed that sleep is for the brain, not the body. Could a shortage of shut-eye also be helping to make us fat? Several large-scale studies suggest there may be a link. People who sleep less than 7 hours a night tend to have a higher body mass index than people who sleep more, according to data gathered by the US National Health and Nutrition Examination Survey. Similarly, the US Nurses Health Study, which tracked 68,000 women for 16 years, found that those who slept an average of 5 hours a night gained more weight during the study period than women who slept 6 hours, who in turn gained more than whose who slept 7. | 
