Chemical imbalances in the brain may be partly to blame for life-disrupting sleep disorders.
Sleep disorders are common
Scientists found evidence linking faulty brain chemistry to two disorders, obstructive sleep apnoea (OSA) and REM sleep behaviour disorder (RBD).
Obstructive sleep apnoea causes breathing to temporarily stop or diminish on many occasions throughout the night.
Symptoms include snoring and excessive daytime sleepiness. It can also affect blood pressure, memory and even reaction time while driving.
Patients with REM sleep behaviour disorder literally act out their dreams during the rapid-eye movement, or REM, phase of sleep, moving their arms and legs, getting out of bed, talking and shouting, and even hitting or punching.
The researchers, from the University of Michigan, studied 13 patients with a rare and fatal degenerative neurological disease called multiple system atrophy (MSA).
MSA is almost always accompanied by severe sleep disorders, and every person who took part in the study had both OSA and RBD.
Comparisons with people who had no sleep problems, showed that the MSA patients had a far lower density of certain types of brain cells that produce the key chemicals dopamine and acetylcholine.
The lower the density of cells, the worse the sleep problems were.
The patients with the fewest dopamine-producing cells in the striatum of their brains had the worst RBD symptoms of thrashing, talking and violent flailing while they slept.
The striatum controls the way the body moves, and an imbalance in dopamine may disrupt this control.
The researchers also found patients with the lowest levels of acetylcholine-producing cells in the brainstem had the most interruptions in their breathing during sleep.
Analysis showed that cells were most likely to be lost in areas of the brainstem that are connected to the part of the brain that controls the muscles of the upper airway and tongue.
Those muscles are crucial to maintaining uninterrupted breathing during sleep.
The researchers stress that their work does not prove that the sleep problems are caused by the low levels of brain chemicals - just that there appears to be a correlation.
Allen Davey, director of the British Snoring and Sleep Apnoea Association, said: "Obstructive Sleep Apnoea is normally considered a physiological condition relating to size, shape and patency (openness) of the upper airway during sleep."
The research is published in the journal Neurology.