Inclusion Criteria:  Age-and sex-matched with MDD+RBD subjects; Free of psychiatric disorders based on M.I.N.I.; Without a personal history or a family history of RBD or neurodegenerative diseases (i.e. dementia and PD) Free of RBD symptoms or RWSA by vPSG  Exclusion Criteria:  Presence of narcolepsy and other neurodegenerative diseases that may give rise to RBD and RWSA; Severe psychiatric illness that could not permit a valid written informed consent or otherwise is not suitable for participating in a study; Use of antibiotics within one month and the use of probiotics within three days prior to sample collection; Pre-existing or post-operation of gastrointestinal diseases (e.g. inflammatory bowel disease, gastrointestinal cancer).