The world's largest sleep study said that sleeping too much could be harmful for cognitive function. The study from the University of Ontario even suggests that too much sleep is just as bad as too little sleep. The study surveyed 40,000 participants from around the world. It found that the ideal amount sleep needed is between seven and eight hours per night. Most people in the U.S. are getting six hours of sleep per night. However, the researchers found that even one night of adequate sleep can improve cognition. Conor Wild, Lead study author, in press release.