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 and infections etc., but the mechanism of BFS is unknown.It's also not known if it is a disease of the motor nerves, the muscles, or the neuromuscular junctions.
(Note: As the name suggests, BFS is not a dangerous condition, but there are other conditions with somewhat similar symptoms that can be serious.)
Further info UK NHS
Rarely, patients can develop more serious twitching and muscle contractions which are known as Cramp Fasciculation Syndrome - it's cause is also unknown. See: Revue Neurologique Volume 161, Issue 12, Part 1,