Nighttime sleep talking refers to unconscious talking, singing, crying, laughing or mumbling during sleep, which is often not recalled after waking up. This is very common, with about half of all children experiencing sleep talking, and about 5% of adults having nighttime sleep talking, with some having a familial tendency. The meaning of sleep talking is not very clear, and some of the content of sleep talking may be related to psychological factors. Frequent sleep talking in children may be due to abnormal neurological development, while in adults it may be due to psychiatric disorders. Nighttime sleep talking may also be associated with some sleep disorders, such as obstructive sleep apnea syndrome, abnormal rapid eye movement sleep behavior, and sleep terrors.