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Insomniacs often have troubles with concentration. While previous studies were unable to find the cause, new brain imaging research may help explain why people with insomnia often struggle to pay attention during the day.

Researchers found that the brains of insomniacs do not properly turn on the regions important for working memory tasks and turn off "mind-wandering" regions irrelevant to the task.

"Based on these results, it is not surprising that someone with insomnia would feel like they are working harder to do the same job as a healthy sleeper," lead author Sean P.A. Drummond, PhD, associate professor in the department of psychiatry at the University of California, San Diego, said in a news release.

The latest study involved 25 people with primary insomnia and 25 healthy sleepers. Participants, who were on average 32 years old, underwent functional magnetic resonance imaging scans while performing a working memory task.

While insomnias did not differ from good sleepers in objective cognitive performance on working memory tasks, MRI scans revealed that insomniacs could not modulate activity in brain regions used to perform the tasks.

The study revealed that as the task got harder, healthy sleepers used more resources within the working memory network of the brain, particularly the dorsolateral prefrontal cortex. However, insomniacs were unable to recruit more resources in these brain regions.  Additionally, as the task got harder, insomnias did not dial down the "default mode" regions of the brain that tend to only be active when people's minds wander.

"The data help us understand that people with insomnia not only have trouble sleeping at night, but their brains are not functioning as efficiently during the day," Drummond said. "Some aspects of insomnia are as much of a daytime problem as a nighttime problem. These daytime problems are associated with organic, measurable abnormalities of brain activity, giving us a biological marker for treatment success."

Around 10 percent to 15 percent of adults have an insomnia disorder with distress or daytime impairment, according to The American Academy of Sleep Medicine.

Insomnia often is a comorbid disorder, meaning that it occurs with another problem like depression, chronic pain or addiction.  Very few insomniacs have primary insomnia, which is defined as a difficulty falling asleep or maintaining sleep in the absence of a coexisting condition.

The findings are published in the journal Sleep.