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By Roger Highfield on

Did you know…seven secrets of the world’s biggest sleep study

Preliminary insights from a pioneering global study of the impact of sleep on our ability to reason and think were revealed recently at a special event in Manchester.

Prof Adrian Owen and I discussed his new book, Into the Grey Zone, in which he describes his research over two decades on patients trapped in the twilight zone of consciousness, between awareness and death.

A huge online study of sleep, an everyday example of this twilight zone, was launched earlier this year his team at the University of Western Ontario, using the website

More than 100,000 people have already taken part and we discussed the first analysis by his team at the latest Manchester Science Festival, in an event in the Museum of Science and Industry.

Data are still pouring in from the study but his colleagues have already sifted online cognitive tests performed by the first 20,000 respondents, who reported on their sleep patterns and lifestyle by answering 30 questions. Here are seven preliminary insights to emerge from his work:

What is the optimum amount of sleep?

The optimal amount of sleep seems to be between 7-8 hours. ‘I don’t think people will be surprised by the main finding which is that your cognition is absolutely at its best with between seven and eight hours of sleep,’ he said. “That is about right. There is something in that old saying about eight hours of sleep being the right amount.’

I asked the jet-lagged professor, what effect does sleep deprivation have? 

‘A large number of people (several thousand), more than you think, reported getting between four and five hours of sleep,’ said Prof Owen. The team compared these people with those who claimed to get 8 hours and the 4-hour sleepers were worse ‘on every single cognitive test’ – when it came to cognitive skills, they were ‘significantly poorer.’

The difference in performance between people who slept four hours and those who slept the optimal amount is 0.34 standard deviations, or equivalent to about 5 IQ points being ‘knocked off’, he said. This is akin to ‘having a couple of drinks’. So far, sleep seems to affect performance on all tests equally, so a poor night’s sleep translates into a poor performance.

Do we understand the link between sleep deprivation and poor performance?

In a recent study on 27 sleeping people, Prof Owen and his colleagues used a method called EEG to study electrical activity in the brain and revealed a link to what are called spindles, bursts of activity in a deep structure called the thalamus,  which plays a central role in memory, attention, perception, awareness, thought, language, and consciousness.

Earlier studies have linked spindles with IQ. “In fact, spindles are the only biological marker of intelligence’, he said, explaining how he finds it remarkable that they can be used to predict how well you do, when wide awake, on a decision-making task.

The new work by his lab connects these ‘squiggles’ of electrical activity during light sleep with reasoning abilities. “We have identified the brain circuits – those linked with higher cognitive functions that were time-locked to spindles that are correlated to cognitive abilities,” he said. So, it seems sleep deprivation may interfere with spindles, ‘damaging our reasoning ability.”

Can you have too much sleep?

Yes. Prof Owen’s huge survey reveals that too much sleep also seems to be associated with poorer cognitive performance. People who got more than 8.8 hours of sleep also tended to do worse at the tests. ‘Too much sleep can also be bad for you.’ However, the effects were not as large as those who were sleep deprived.

Is age a factor in how much you need?

Though early days, the 7-8 hour recommended amount of sleep remains the same, even though the amount of sleep changes over a normal lifespan. ‘It is still the people who get seven to eight hours who do the best,’ he said.

Has your sleep study shed light on other aspects of intelligence?

Some more general insights into cognition have emerged. Many scientists believe that if you learn a second language, particularly as a child, it will ‘somehow make you smarter,’ he said. Data on 5000 or so people who could speak two languages suggested they are no different from those who only speak one. ‘That’s not to say there aren’t advantages to speaking two languages, but it doesn’t make you smarter.’

What is sleep? 

Sleep marks a shift in consciousness, explains Prof Owen. ‘When you go to sleep at night, there are some parts of your brain that continue to process information around you, just as if you are awake, and other parts of your brain which shut down and don’t process that at all – it is a change of the level of your consciouness.’

Sleep is as vital for life as food or water. Lab rats deprived of sleep die within a month, and too little affects your immune system and appetite, is linked to heart disease, obesity, and diabetes. Increasingly, a lack of sleep is implicated in mental health problems including bipolar disorder, depression and schizophrenia, even Alzheimer’s

During the event, we also discussed the world’s largest intelligence test and the subject of his book,  research that Prof Owen began in Cambridge two decades ago, notably at the MRC Cognition and Brain Sciences Unit, using brain scans to investigate patients who are trapped in the grey zone between life and death.

Because of the rise in recent decades of resuscitation and emergency medicine, which can save the lives of severely-brain damaged patients, tens of thousands of people around the world now inhabit the grey zone.


A recording of the event can be heard here