What you are describing is known as the “hypnagogic jerk” which is an involuntary muscle twitch as a person is falling asleep. Hypnagogic jerks occur due to a mismatch of brain activity and muscle relaxation. Sometimes during the first stage of sleep, the muscles relax before the brain enters deeper sleep. This the causes the brain to send out a signal to tense the muscles and results in a free falling sensation followed by a hypnagogic jerk. These jerks are considered normal and sleep deprived individuals tend to experience these jerks more frequently.