Sleeping in on your days off may extend your life, according to a study.Researchers found people who got less sleep during the week, but slept in on weekends had the same mortality rate as people who got seven hours of sleep each night.Over a decade was spent following nearly 44,000 people in Sweden to come up with the findings.There's no obvious answer on why this makes a difference. One professor from the Stress Research Institute at Stockholm University thinks people are possibly catching up on lost sleep.The study was published in the Journal of Sleep Research.