Think the more you sleep, the better you’ll perform? Unfortunately, that’s not how it works. A new study of 40,000 people around the world has found that people who regularly slept more or less than seven to eight hours a night had impaired cognition, including the ability to store and recall information, solve problems and communicate. They also had problems with reasoning and verbal abilities.

So, everyone who’s been recommending seven to eight hours of sleep — which the study refers to as a good night’s sleep — has been right, concludes the study’s lead author, Conor Wild, PhD, a professor of neuroscience at the Brain and Mind Institute of Canada’s Western University. According to his findings, clocking in these hours, on an average, is all your brain and body need to keep performing their best, regardless of age. People who don’t get this simply can’t focus on the task at hand and often perform worse across all cognitive domains.

Confusingly, however, if you’re looking to improve your thinking and performance ability, a little extra sleep does help. Cognition improves with as little as one night of good sleep, Wild’s study found. But consistently getting more than eight hours’ sleep backfires.

Wild’s sleep study was launched in June 2017 with more than 40,000 people from around the world answering an in-depth questionnaire about their sleep habits and undergoing a series of cognitive performance activities. Participants also reported which medications they take regularly, how old they were, where they live, and what kind of education they’d received, because these are all factors that might affect sleep.

Bottom line? One night of extra sleep can kick you into high gear the next day, but consistently getting too much sleep takes a toll as much as too little sleep.