Benign fasciculation syndrome (BFS) is a neurological disorder characterized by fasciculation (twitching) of various voluntary muscles in the body. The twitching can occur in any voluntary muscle group but is most common in the eyelids, arms, legs, and feet. Facial twitching almost always affects just one side of the face - usually the left side.
Various factors can trigger or exacerbate BFS, including anxiety, stress, medicinal drugs, recreational drugs, vitamin deficiencies infections etc. but the mechanism of BFS is unknown, and it is not known if it is a disease of the motor nerves, the muscles, or the neuromuscular junction. Note: refs needed