['Rapid eye movement (REM) sleep behaviour disorder (RBD) is a common sleep disorder that can be associated with a number of neurodegenerative conditions as well as with narcolepsy. Current diagnostic criteria require overnight polysomnography, and there are no other biomarkers available. The control of REM sleep is complex with a putative on/off switch within the brainstem activated, amongst other things, by hypocretinergic pathways from the lateral hypothalamus.', 'Cerebrospinal fluid hypocretin levels were measured in five patients with idiopathic RBD.', 'Hypocretin levels were between 254 and 450 pg/ml and therefore within the normal range of >100 pg/ml.'] Are cerebrospinal fluid hypocretin levels normal in idiopathic REM sleep behaviour disorder?