I’m fascinated by studies that look at sleep in different cultures and societies. I’m also interested in studies that investigate sleep in the “real world,” as opposed to exclusively from within the scientific sleep laboratory.
Don’t get me wrong. Laboratory sleep research is of vital importance. There are innumerable factors scientists can identify and control from within the sleep lab that that couldn’t possibly happen in people’s everyday environments, or by relying on people’s self-reported information about sleep. But a sleep laboratory by nature is an artificial environment. And insight we can gain about sleep in our natural settings and routines is also important and revealing.
A new, large-scale study does both. This study, dubbed by scientists as the “world’s largest” sleep study to date, included a global sample of more than 10,000 people, and captured information about the impact of real-world sleep habits on how we think, feel, and function.
Analyzing sleep in the real world, and around the globe
Scientists at Canada’s University of Western Ontario used the Internet to recruit thousands of participants from around the world. Questions about participants country of origin were optional, so it’s not possible to know the full geographic scope of participation. A majority of participants came from the UK, Canada and the US. But study participants also came from India, Australia, New Zealand, Pakistan and at least half a dozen European nations.
More than 40,000 people registered to participate in the study. That’s a pretty incredible number. I like to think it’s a sign of how much people are interested in sleep these days! The final study group included 10,886 people between the ages of 18-100, with an average age just under 42. Participants were split roughly 60-40 percent between female and male, respectively.
All participants completed a questionnaire, sharing information about themselves and their backgrounds, as well as detailed information about their daily lives, including how often they experienced depression and anxiety. And they shared information about how much they sleep.
Also online, participants completed a series of 12 tests, which measured different types of cognitive function, including:
- Short-term and working memory
- Strategic thinking and problem solving
- Attention and concentration skills
- Visual and spatial processing and planning skills
- Reasoning skills
- Communication skills
To assess how individuals’ sleep affected their performance, everyone participating in the study was asked to report how much sleep they’d gotten the night before taking the tests.
Sleep scientists were going in search of a deeper understanding of how everyday sleep patterns affect waking cognitive performance, in skill areas we rely on constantly, all throughout the day.
They found that a full half of their more than 10,000 worldwide volunteers were sleeping less than 6.3 hours a night. (That’s even higher than recent research by the U.S. Centers for Disease Control, which found nearly 30 percent of U.S. adults sleeping no more than 6 hours a night.)
They also made some interesting discoveries about how sleep duration can help—or hinder—our cognitive abilities, including what the optimal range of sleep is for cognitive performance, and what types of cognition are most affected by the chronic sleep debt that so many people carry with them in their daily lives.
Disordered sleep affects cognitive abilities—but not evenly
The takeaway: Routinely getting 7-8 hours sleep is one way to ensure you’re able to perform at your best in all the thinking, planning, decision-making and communicating you do throughout your day.
Based on the analysis of its participants information and testing, scientists in this study identified the optimal sleep amount for overall cognitive function: 7-8 hours a night. Sleepers within that window demonstrated the strongest performances on cognitive tests.
Researchers found that sleeping outside that 7-8-hour window affected some cognitive skills more seriously, while leaving others relatively unaffected.
Reasoning skills and verbal skills, as well as overall cognition worsened for participants whose sleep fell out of the 7-8-hour range.
Short-term memory, on the other hand, was largely unaffected by sleep amounts, according to the study findings.
Before you go thinking your short-term memory isn’t vulnerable to the negative effects of poor sleep, keep in mind: this study captures a snapshot of the effects of sleep duration on memory and cognitive function. It does not measure the cumulative, long-term effects of too much or too little sleep on cognition or memory. We’ve seen a fast-growing body of research showing the effects of sleep on risks for Alzheimer’s and other neurodegenerative diseases.
Both too little and too much sleep leaves us cognitively struggling
The takeaway: Sticking to a consistent sleep routine is the single most significant step you can take to protect and improve your daily performance, as well as your health. It can drastically reduce—or even eliminate—the need to play catch up with your sleep. That’s going to pay dividends for your productivity, your communication skills, and your ability to make decisions and problem solve.
The study found the 7-to-8-hour window of sleep to be optimal for cognitive performance. What happened for the people who slept more or less than that amount?
Researchers discovered that sleeping more or less than 7-8 hours a night had a negative effect on cognitive function. Interestingly, they found almost no difference in the degree of impairment. Sleeping less than 7 hours, or more than 8, had an almost identically negative impact on cognition—and the further participants moved away from that optimal sleep zone, in either direction, the worse cognitive performance became.
Oversleeping is an often-overlooked sleep issue. I wrote recently about the problem of oversleeping (also known as hypersomnia). Oversleeping has strong links to low mood and depression, and is a symptom of several sleep disorders, including obstructive sleep apnea, restless leg syndrome, and narcolepsy. This new research strongly suggests oversleeping impairs our ability to reason and communicate clearly, quickly, and effectively—an important avenue for additional future research.
One night of good sleep makes a difference
The takeaway: You can make improvements to your cognitive performance by just a single night of additional sleep. If you’re presenting at a meeting, or interviewing for a job, you can increase the chances you’ll be at your sharpest and most articulate by making extra time for sleep the night before. Just don’t go overboard and sleep 10 hours the night before—as this study shows, that’s as likely to make your brain sluggish as sleeping 5 hours a night.
I get asked this question a lot: how much difference does one single night of sleep really make? In the results of this latest, large-scale study, the answer was: a real difference.
Scientists found that among people who weren’t getting enough sleep, increasing sleep duration to within the 7-8 hours range for a single night led to stronger cognitive performance the next day. Scientists also saw results that indicated a single night of sleeping less than 7-8 hours led to a drop in cognitive performance.
Lack of sleep ages our brain function
The takeaway: Don’t try to be a super sleeper. You may think you’re maximizing your productivity, or squeezing the most out of life, but all that extra time is coming your way at a steep cost. Getting by on a handful of hours of sleep a night is no way to live. It will hurt you physically and emotionally. As this study suggests, it will also age your brain performance.
This was one of the study findings that really jumped out at me. Among the study participants who were severely sleep deprived—in this case, that means sleeping no more than 4 hours a night—their cognitive impairment was equivalent to adding almost 8 years to their age. That’s right: sleeping 4 hours (or less) “aged” brain performance by close to a decade.
That’s serious cognitive impairment stemming from a single night of poor sleep. We know sleep ages us in a number of ways. Lack of sleep lowers the body’s production of human growth hormone, which helps slow the body’s aging process and affects everything from the aging of our organs to the number of wrinkles we have. Poor sleep accelerates the aging of the immune system, and makes us more prone to chronic, age-related diseases including type 2 diabetes and cardiovascular disease. And there’s compelling evidence that lack of sleep—even a single night of it—increases levels of the damaging brain proteins that are linked to Alzheimer’s disease.
Sleep matters to brain function at every age
The takeaway: If you’re 28, don’t assume you can get away with sleeping for a handful of hours a night and still function at your best. If you’re 68, don’t assume you need less sleep than you did when you were in your 20s or 30s.
One of the advantages of a study that included volunteers ranging in age from 18-100? It gave researchers a chance to examine how age affected the relationship between sleep duration and cognitive function.
Researchers found NO difference in the effect that sleep amounts had on cognitive performance, based on age. The 7-8-hour sleep range is optimal for cognitive function at every age. And sleeping too much or too little had the same negative consequences for everyone, regardless of age. According to these results, sleeping 4 hours or 10 hours has the same negative cognitive impact whether you’re 25 or 75.
Surprised? There’s at least one caveat to keep in mind. The number of participants over the age of 70 was very small (269 volunteers out of more than 10,000). This study, as large as it is, may not include a population best suited to make an assessment of how age affects sleep’s impact on cognition.
There is ample evidence that suggests sleep patterns change with age. Sleep disorders become more common as we age. Hormones that regulate sleep, and other physiological functions that affect, sleep shift and decline, making sleep disruptions more likely. And overall, studies indicate that sleep duration declines with age.
But what science doesn’t show is that our need for sleep changes with age. This study, and others, suggest the opposite: that throughout our adult lifespan, we continue to need the same amount of sleep to function and feel at our best.
We’ve seen very few studies that examine the relationships among age, sleep duration, and cognition. (According to the current study’s researchers, there is only one other besides their own.) Understanding this relationship—among age, sleep and cognitive function—is of vital importance.
Michael J. Breus, PhD, DABSM
The Sleep Doctor