Original: In particular, low levels of melatonin may stop the depolarization current in the nerves, preventing the stimulation of the muscles and any consequent enactment of the dream activity by the body (e.g. preventing a sleeper from flailing his legs when dreaming about running).
Simplified: In particular, low levels of melatonin may stop the depolarization current in the nerves, which prevents the stimulation of the muscles.