A little-known and poorly understood sleep disorder that occurs during the rapid eye movement, or REM, stage of sleep is believed to foreshadow neurodegenerative brain diseases such as Parkinson’s disease and dementia with Lewy bodies.

The disorder, known as REM sleep behavior disorder, or RBD, affects around 1% of the general population worldwide and about 2% of adults over 65.

RBD is not well understood. In some cases, a clear cause cannot be identified. In other cases, the disorder may be caused by something specific, such as obstructive sleep apnea, narcolepsy, psychiatric disorders, use of antidepressants, autoimmune disorders and brain lesions, which are areas of damaged brain tissue.

In both situations, REM sleep behavior disorder may be associated with synucleinopathies, a group of neurodegenerative disorders in which aggregates of the protein α-synuclein accumulate in brain cells. The most common of these neurodegenerative disorders is Parkinson’s disease and may be the disease’s first symptom.

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