Rapid eye movement sleep behaviour disorder (RBD) occurs when the normal mechanism which induces paralysis during rapid eye movement (REM) sleep is faulty.
Patients with the condition frequently act out dreams and can harm themselves or their spouses.
RBD often occurs in patients with neurodegenerative illness and previous studies have suggested that it may precede other neurological symptoms.
Researchers have now found that nearly two thirds of patients with RBD will develop a degenerative brain disease within 11 years.
The study, presented this week at the Associated Professional Sleep Societies' SLEEP 2006 meeting in Salt Lake City, US, included 23 patients who had been diagnosed with idiopathic RBD between 1988 and 1995.
After an average of 11 years' follow up, one patient developed Parkinson's, four had dementia while another 10 reported neurological symptoms highly suggestive of parkinsonism or dementia.
Lead researcher, Dr Maja Tippmann-Peikert, a neurologist at the Mayo clinic's Sleep Disorder Center in Rochester, Minnesota, said excessive nocturnal behaviours and dream enactment should be noted.
'Hopefully, early identification of patients with idiopathic RBD will lead to close monitoring and early treatment of any developing neurological disorders,' she said.
Meanwhile, another study presented at the conference linked early development of RBD with antidepressant use.
Researchers compared 22 idiopathic RBD patients aged under 50 with 22 age- and sex-matched controls. They found that 86 per cent of the patients had a history of antidepressant use compared with 27 per cent of controls.
The antidepressants included SSRIs, venlafaxine, mirtazapine and tricyclic anti-depressants.
Lead researcher Dr Robert Auger, a psychiatrist from the Sleep Center at the Mayo Clinic, said: 'Physicians should be aware of this potential side-effect, particularly in patients who complain of sleep disturbances.'