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Maggie McTavish, 26, doing her daily routine of meditating, Oct. 7, 2018.Michelle Siu

Maggie McTavish now thinks more clearly than she has in years. She’s more productive at work. She’s focused and alert. Her mood has improved – all due to one lifestyle change: She began sleeping at least seven hours a night.

“Almost my entire life, it’s been always all about work and trying to do as much as I can in a particular day,” says Ms. McTavish, 26, who owns a digital-marketing agency in Toronto.

Until last year, that meant getting by on an average of four to six hours of slumber a night.

"I wasn’t aware that sleep was really essential to my life,” she says. "It was always at the bottom of the list.”

It turns out many of us, too, could be a lot sharper, if only we’d get the right amount of shut-eye.

A new large-scale study suggests sleeping too little – and too much – can dull your thinking. The findings, published in the journal SLEEP on Monday, show people who get an average of seven to eight hours of sleep per night perform better on cognitive tasks, such as reasoning and problem-solving, than those who sleep more or less than this generally recommended amount.

Moreover, those who sleep less than four hours a night showed impairment equivalent to adding almost nine years to their age. Yet a large number of people are not getting nearly enough sleep. Half of all participants reported sleeping less than 6.4 hours per night – and it reflected in their test scores.

The study, conducted by neuroscientists at the Western University, adds to growing evidence of the importance of sleep, not just for overall physical health, but brain health and function.

The idea that seven to eight hours is an optimal amount is "not terribly surprising to people,” says co-author Dr. Adrian Owen, a professor of cognitive neuroscience and imaging at Western. But, he says, "What might be surprising is almost nobody is doing that.”

The researchers recruited more than 40,000 online participants from around the world.

Volunteers were asked to complete a questionnaire about their personal information and sleep habits, including the average number of hours they slept per night over the past month. Then, more than 10,000 participants were given a series of tests that measured their cognitive function, such as their short-term memory, attention and deductive reasoning.

As Dr. Owen explains, habitually getting inadequate sleep did not appear to affect participants’ memory very much, but it did seem to hamper their ability to reason and solve problems.

Participants who slept four hours a night showed equivalent cognitive function to those almost nine years older – not a good sign for the likes of the late British prime minister Margaret Thatcher and U.S. president Donald Trump, both famous for needing only around four hours of sleep, Dr. Owen says.

"You think you can run a country on four hours of sleep? That’s fine. But you’re running a country [as someone] nine years older than you actually are,” he says.

One finding that stood out, he adds, is that getting too much sleep is also detrimental. He notes this may have to do with a phenomenon called "sleep inertia.” Sleep inertia can be experienced as the grogginess one feels after a long, deep slumber.

Professor Teresa Liu-Ambrose of the University of British Columbia, who was not involved in the study, says the latest findings align with previous sleep research. Authorities such as the American Academy of Sleep Medicine, for instance, recommend at least seven hours of sleep per night for adults.

Dr. Liu-Ambrose, the Canada Research Chair in physical activity, mobility and cognitive health, notes, however, that while these kinds of recommendations provide general guidelines, the amount of sleep needed varies among individuals.

She adds there is more to a good night’s sleep than just duration. Other important factors include how quickly people fall asleep, the number of times they wake in the night, and whether people are sleeping as deeply as they should be when they cycle through the phases of sleep, she says.

A good indicator of how well you’re sleeping may be whether you wake up feeling refreshed, rather than how many hours you spend in bed.

Dr. Liu-Ambrose explains researchers know sleep is critical for consolidating memory, which is the process of forming long-term memories. And they’re learning it is also necessary for removing toxins within the brain, namely the protein amyloid, associated with Alzheimer’s disease.

"I think we can all recover from … acute episodes of sleep deprivation,” she says. "But certainly from a chronic perspective, it’s probably not very beneficial to the brain.”

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