Ten hours of sleep at once may indeed recharge us, and allow us to perform well for several hours after waking, according to research recently published in Science Translational Medicine. But "the brain literally keeps track of how long we've been asleep and awake--for weeks," says Harvard Medical School (HMS) neurology instructor Daniel A. Cohen, M.D., lead author of the study. And that means that the bigger our aggregate sleep deficit, the faster our performance deteriorates, even after a good night's rest.