Why do my muscles twitch and flex on their own sometimes?

See below: There are many causes of muscle twitching, most benign and occasionally but rarely serious. The most common reasons are overwork of weak muscles, too much caffeine or other stimulants, too little sleep. Other causes are electrolyte imbalances, liver problems, thyroid problems and certain neurological diseases. If the twitching does not go away, you should see your doctor.