This is Scientific American ― 60-Second Science. I'm Karen Hopkin.Few things are more satisfying than sleeping late on weekends.But though the extra z's may improve your mood, they do not appear to improve your health.Because a new study shows that so-called "recovery sleep" cannot reset the body's metabolic clock...and may actually lead to some serious health issues."Sleep loss can impact a range of physiological systems.It can increase our risk for cardiovascular disease, it can cause weight gain,it can decrease insulin sensitivity, so it can increase our risk of diabetes...."Christopher Depner, an assistant professor in the department of integrative physiology at the University of Colorado, Boulder."And this can happen, depending on what you're looking at, this can happen as quick as just one or two nights of not getting enough sleep."A lot of us shut off the alarm on Saturday and Sunday.But we go right back to burning the candle at both ends once the workweek begins."So we were really interested in how the sort of cycle of between insufficient sleep, weekend recovery sleep, insufficient sleep,can impact your risk of obesity and metabolic disease."Depner and his colleagues invited volunteers to a nine-day snooze-a-thon.One group was allowed to get a full night's sleep. The next was kept to just five hours each night.And the third group went back and forth, restricted to five hours of shut-eye during the workweek,allowed as much sleep as they wanted over the weekend, and then back to five hours for the last couple days.The results?