MONDAY, Nov. 3 (HealthDay News) -- Adults with primary insomnia have a specific neurochemical imbalance that makes it more difficult for their brains to settle down for sleep, a new study says.

People with primary insomnia for more than six months have 30 percent less gamma-aminobutyric acid (GABA), a chemical that slows overall activity in many brain areas, according to the report in the Nov. 1 issue of Sleep. A "racing mind" and an inability to shut down at night is a common complaint of people with primary insomnia.

"GABA is reduced in the brain of individuals with insomnia, suggesting overactivity is present not only at the level of excessive thoughts and emotions, but can also be detected at the level of the nervous system," principal investigator John Winkelman of Brigham and Women's Hospital, in Boston, said in an American Academy of Sleep Medicine news release.

While most insomnia occurs in conjunction with another illness or disorder, either physical or mental, or as a side effect of certain medications or substances, primary insomnia occurs without a coexisting condition. Approximately a quarter of people with insomnia suffer from this particular type of the sleep disorder.

The finding suggests that primary insomnia is a manifestation of hyperarousal, a neurobiological state, that Winkelman said helps validate an often misunderstood complaint of insomnia.

"Recognition that insomnia has manifestations in the brain may increase the legitimacy of those who have insomnia and report substantial daytime consequences," he said. "Insomnia is not just a phenomenon observed at night, but has daytime consequences for energy, concentration and mood."

Lower brain GABA levels also have been found in people with major depressive disorder (MDD) and anxiety disorders. As primary insomnia is a critical risk factor in these conditions, the findings could mean GABA deficiencies seen in people with mood and anxiety disorders may be tied to sleep disturbances.

Many of the hypnotic medications most effective in treating insomnia are benzodiazepine receptor antagonists (BzRAs), which increase activity at the GABA neurons, the study also reported.

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