Oswald (as related in his book Sleep) observed active rocking in some children and adults during sleep. These rocking movements were very powerful, occurring at a rate of one per second during REM sleep. They occur more frequently in the blind or to people institutionalised during childhood. As rocking is a movement used by primates and humans to comfon themselves during stress or unhappi- ness, it is possible that the rocking accompanies disturbing dreams.
The dream probably evokes unhappy feelings or conditions from the past, as the person used rocking to cope with their past distress, it occurs as an habitual accompaniment of the distressing dream. ... A Guide to Dreams and Sleep Experiences