Too Much Sleep br Could Be Bad for You The world's largest sleep study said that sleeping too much could be harmful for cognitive function. The study from the University of Ontario br even suggests that too much sleep is just br as bad as too little sleep. The study surveyed 40,000 participants from around br 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 br six hours of sleep per night. However, the researchers found that even one night of adequate sleep can improve cognition.