Children with attention deficit disorder have brains that cannot stop daydreaming, claims a new study.
Researchers found that they physically find it harder to switch off a "default setting" of the brain designed to pass the time when not focused on a task.
This means their thoughts are more likely to wander at random or daydream when they should be concentrating.
The study, by the University of Nottingham, may explain the physical process behind ADHD sufferers lack of ability to concentrate on the job at hand.
"You could say that children with ADHD are easily bored but this shows there is a biological basis to it," said the study leader Professor Chris Hollis.
Children with ADHD are excessively restless, impulsive and distractible, and experience difficulties at home and in school.
Although no cure exists for the condition, symptoms can be reduced by medication and behavioural therapy.
The drug methylphenidate – more often known by the brand name Ritalin – is commonly used to treat the condition.
Using brain scans the researchers have shown that children with ADHD have difficulty in "switching-off" the default mode network (DMN) in their brains.
This network – which connects various parts of the brain – is usually active when we are doing nothing, giving rise to spontaneous thoughts or "daydreams", but is suppressed when we are focused on the task before us.
In children with ADHD, however, it is thought that the DMN may be insufficiently suppressed on "boring" tasks that require focused attention.
Using a video game, researchers compared brain scans of eighteen children with ADHD, aged between nine and 15 years old, against scans of a similar group of children without the condition.
The children with ADHD were tested when they were taking their methylphenidate and when they were off their medication.
By studying the brain scans, the researchers were able to show that typically developing children switched off their DMN network whenever they saw an item requiring their attention.
However, unless the incentive was high, or they had taken their medication, the children with ADHD would fail to switch off the DMN and would perform poorly.
Dr Martin Batty, co-author of the study, said: "Using brain imaging, we have been able to see inside the children's heads and observe what it is about ADHD that is stopping them concentrating.
"Most people are able to control their 'daydreaming' state and focus on the task at hand.
"This is not the case with children with ADHD.
"If a task is not sufficiently interesting, they cannot switch off their background brain activity and they are easily distracted. Making a task more interesting – or providing methylphenidate – turns down the volume and allows them to concentrate."
The findings are published in the Journal of Child Psychology and Psychiatry.