16-year-olds did best in exams when they slept for 7 hours London - Arabstoday They often protest that they need the extra hours in bed. But getting too much sleep could in fact be leaving teenagers behind at school, according to research. A study found that they performed better academically if they cut down on lie-ins and had more regular sleep patterns. And while many think they need nine hours, the optimum amount of sleep for a typical 16-year-old is just seven a night, according to scientists at Brigham Young University, in Utah. Eric Eide, study author, said: 'We're not talking about sleep deprivation. The data simply says that seven hours is optimal at that age.' Reporting the findings in the Eastern Economics Journal, the team said the right amount of sleep decreases with age. The optimum amount of sleep for a ten-year-old is around nine hours while for 12-year-olds it is eight hours. The researchers studied 1,724 primary and secondary school students across the U.S., assessing how much sleep they got and how they scored in standardised tests. The impact of sleep on test scores depended on a number of factors, such as how educated the child's parents were. Economics professor Mark Showalter, who co-wrote the study, added: 'Most of our students at BYU, especially those that took early-morning seminary classes in high school, are going to realise that nine hours of sleep isn't what the top students do.' According to researchers at the University of Chicago, those who sleep for less than five hours a night for periods of longer than a week have significantly lower levels of testosterone than those who get a full night's rest. And with testosterone affecting men's libido and energy levels, those who miss out on sleep are much more likely to be put off sex. The affect is so drastic it reduces the hormone to levels more akin to someone 15 years older, they reported in the Journal of the American Medical Association. Meanwhile, a 10-year study of thousands of women found significantly fewer cases of cancer, including breast cancer, among those who were the most physically active. But sleeping less than seven hours a night wiped out the benefits of exercise and increased the risk of cancer. The researchers, from the U.S. government-funded National Cancer Institute, asked almost 6,000 women about their exercise and sleep routines before tracking their health for 10 years.