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People who sleep well are able to selectively activate and deactivate regions of the brain used for "working memory" when trying to complete complex tasks, but primary insomniacs struggle to block out irrelevant thoughts.

Researchers from the University of California compared brain scans of 25 patients against 25 healthy sleepers, and found that while both were equally competent at completing tasks involving working memory, those with primary insomnia could not "dial down" parts of the brain which are activated when the mind wanders.

Dr Sean Drummond, who published his team's findings in the Sleep journal, explained: "People with insomnia not only have trouble sleeping at night, but their brains are not functioning as efficiently during the day.

"We found that insomnia subjects did not properly turn on brain regions critical to a working memory task and did not turn off 'mind-wandering' brain regions irrelevant to the task.

"It is not surprising that someone with insomnia would feel like they are working harder to do the same job as a healthy sleeper."