Sleepiness has long been an issue among adolescents. One study of middle school students found that "delaying school start times by one hour, from roughly 7:30 to 8:30, increases standardized test scores by at least 2 percentile points in math and 1 percentile point in reading."
But it's not just kids. Short-term memory is a crucial component of learning, and sleep deprivation significantly impaired the ability of adult volunteers to remember words they'd been shown the day before. In another study, researchers found that while people tend to improve on a task when they do it more than once, this isn't true if they are kept awake after they try it the first time — even if they sleep again before doing it again.