At quarter-to-11 on Christmas Eve last year, my two little kids were tucked up in bed, the presents were wrapped, the dishwasher on, and I sat down with a whiskey for the final task of the evening: taking a brain test. For the next 30 minutes, tired and slightly tipsy, I ran through a battery of strangely addictive video games: trying to remember strings of numbers, figuring out which pattern in a series doesn't belong, and quickly identifying the colour of ink used to spell out the words "red" and "blue" flashing on my screen (which is maddeningly hard to do).
I had signed up, with equal doses of curiosity and civic-mindedness, to play guinea pig for what has been billed as the "world's largest sleep study," run out of Ontario's Western University. Along with roughly 40,000 other people, I answered a short survey about things like my education, general sleep patterns, and mental health, and strained my brain for three nights in a row on a dozen tried-and-tested games that assess "cognitive performance," to see how my sleep—or lack of it—affected my smarts.
Nearly a year later, the first results are in. This month, the research team behind that effort published results from 10,000 of the volunteers (a lot of the data had to be set aside because the participants were too young, or their surveys incomplete, but that's still the largest study on sleep and cognition).
As any sleepless parent might predict, people who slept less than seven or eight hours in a night took a hit on their brain performance. Strikingly, sleeping four hours or less a night seemed to have about the same impact as aging nine years. But, surprisingly, people who slept more than the recommended amount were just as impaired. And that was true no matter how old the participants were. "That was interesting," says Conor Wild, lead author on the work.
Like me, Wild has a good reason to be interested in sleep: he describes himself and his wife as the proud parents of two-year-old twins and a four-year-old. "When the twins were born, and we were both up at 4 a.m. feeding a baby, we said, 'What is this doing to us?'" he laughs. "It's pretty intense."
Although sleep is such a huge part of our lives, surprisingly little is known about the details of how and why sleep affects our brains and health; even the over-arching question of why we sleep still goes unanswered. "That's the Holy Grail: what is the purpose of sleep? A hundred different people have a hundred different answers," says psychologist and neuroimager William Scott Killgore at the University of Arizona, who wasn't involved with the new work. "Maybe it's just an efficient thing to do when it's dark, so you don't step in a hole."
Says John Noel, executive director of the Illinois-based Sleep Research Society: "We probably don't even know as much as we thought we did, once upon a time.
"There are so many aspects to sleep."
Getting people into a lab for sleep studies is logistically hard, limiting the number of people who are run through such tests; Killgore typically studies about 50 people over a year, not tens of thousands. And most studies of sleep and brain performance, says Wild, have been done on full-throttle sleep deprivation: assessing how much going without sleep for days on end affects our ability to pay attention. Much less is known about "chronic sleep restriction"—the practice of getting a little less sleep than we'd like day after day. Too much sleep has been linked to medical problems from diabetes to heart disease, but it's hard to untangle which is the cause and which is the effect, or whether depression or poverty is at the root of both.
The Brain and Mind Institute where Wild works at Western University decided to tackle these questions by gathering as much data as possible, using an online survey designed to be quick and easy—just a short survey plus 30 minutes of games, three nights in a row. "We had such an enthusiastic response that our servers literally couldn't keep up," Wild says.
Noel argues this sort of study is important: "The more people we can involve, the more we can understand about what's happening in the general public," he says. "That's the kind of thing we need."
The results, published in the journal Sleep, showed a strong link between hours spent in bed and tasks relating to higher brain functions, like problem solving. Those who slept seven to eight hours on average did best out of the group, overall; and those who slept more or less than their own usual on any given night tended to do worse on their tests the next day. Surprisingly, the pattern was the same for all ages; both 70-year-olds and 20-year-olds needed the same amount of sleep, and took the same hit when they slept more or less.
There could be all sorts of explanations for this. Maybe people who, on average, sleep too little, have stressful jobs that pre-occupy them; maybe people who sleep too much have some sort of illness that affects both their bodies and brains. Or maybe it's the sleep itself that causes the effects. Killgore speculates that if one purpose of sleep is for the brain to "cull all the weeds" of unnecessary memories, clearing out space in the brain for more important stuff, then perhaps sleeping too long results in "over-cleaning," like "Oops, I took out a few flowers with the weeds," he laughs.
Wild's massive dataset is impressive, says Killgore. But, he adds, the data could be "dirty:" "They could be eating dinner or watching TV while doing these tests; you don't know."
My own test results, I'm sure, were a little sullied—by alcohol, and kids yelling, "What are you doing, mommy?!" at me -while I was taking them. My sleep tends to be unpredictable and odd, broken into pieces by midnight demands for drinks of water, and cat-naps after reading bedtime stories. There wasn't a way for me to report these details.
Studies like these aren't designed to determine cause-and-effect, or to drill into details. The benefit of having tens of thousands of participants, says Wild, is that the glitches come out in the wash, leaving broad correlations that they can follow up on more. "We have seen an interesting pattern, so now we can dig into it," he says.
In the meantime, does Wild get his recommended eight hours of sleep? "Definitely not," he laughs. Maybe when the kids are older.