People who slept this much (and no more) had top cognitive performance

During sleep, cerebral spinal fluid courses through the brain more rapidly, cleansing it of the waste products naturally produced by brain cells. Muscle is also built while you snooze, through the release of growth hormones, whereas the heart and lungs take a break.

As our understanding of sleep increases, research has sought to explain sleep-related behaviors and explore sleep needs. For instance, one study explored whether there was a biological reason behind teens’ wanting to sleep in, concluding that most teens needed more sleep, to maximize cognitive performance and decision-making. Drawing on this and similar studies, the common wisdom seems to be that more sleep is better.

For teens in particular, this would seem to make sense: their bodies are growing, their emotions are turbulent, their brains are processing increasingly complex social and academic information. But while cognitive performance and decision-making improve in the absence of sleep deprivation, the drawbacks of too much sleep have been left largely unexplored.

UCSF’s Anne Richards’ collaborative study, “Sleep and Cognitive Performance From Teens To Old Age: More Is Not Better” measured cognitive performance compared to average sleep across various ages, and its conclusions turn the more-is-better assumption on its head. Yes, sleep deprivation resulted in lower cognitive performance, but too much sleep had a similar effect. Getting 10 hours of sleep was associated with mental performance on par with 5 hours.

Richards and colleagues’ sleep study used data collected over 20 months from Lumosity.com, a cognitive training website whose users reported their usual sleep duration before playing games based on standardized cognitive tests. The study assessed scores relative to habitual sleep from three games: Speed Match (which measures working memory and processing speed), Memory Matrix (which tests short-term visuospatial memory), and Raindrops (which challenges mental arithmetic in a timed environment).

Perhaps surprisingly, the findings indicated that there was a magic number for how much sleep optimized cognitive function, and it persisted across all ages: 7 hours of sleep was best. As Richards and team noted, peak performance at 7 hours of sleep was “remarkably consistent across younger and middle-aged age groups and is reliably reproduced in different tasks” though the peak was imperceptible in 75–89 year olds. Interestingly, for each hour of sleep over 7 hours, teens showed comparable or steeper drops in performance than 20–24 year olds, suggesting that teens may not benefit from more sleep after all.

Richards et al. note that while other studies have found older people’s cognitive functioning to decline with more sleep, this association has typically been chalked up to underlying health issues. The decrease in cognitive function when teens got more sleep contradicted the assumption that similar declines in the elderly were due to underlying disease or illness, since teens generally have few underlying health issues.

Not only did 7 hours appear to be the correct dosage for all age groups, but Richards and team’s study tracked cognitive performance versus sleep while also controlling for gender and education level. They were able to do this because they were armed with data — lots of data — from Lumosity, whose users provide gender and education information. (All data provided to researchers is first anonymized.)

The authors analyzed 499,273 Speed Match game plays, 447,665 Memory Matrix game plays, and 231,658 Raindrops game plays. The size of the dataset is partly attributable to the cognitive tests being translated into games “designed to be visually appealing and engaging to an internet audience.” So, to control for long-term engagement, the authors looked at a person’s results only the first time they played a game, eliminating any practice effect.

Cognitive test results in the hundreds of thousands suggest that the study’s conclusions are generalizable, as the paper’s discussion section notes: “The great strength of these findings is that they are derived from an unparalleled sample size and represent an impressive range of ages. This generates the statistical power to … identify a small, but precise, relationship between sleep duration and cognitive performance.”

Still, it doesn’t mean that a 7 hour recommendation should be applied to everyone: sleep needs vary within and across individuals. But, if 7 hours of sleep benefits your ability to think and be mentally present, that may indicate that you have healthy, typical sleep regulation.