When the muscles tighten up involuntarily, it is called muscle twitching. It is quite common in eyelid, calf or thumb muscles. It can be caused by many factors like dehydration, strenuous exercise and deficiency of minerals and vitamins.

Other causes of muscle twitching include damage to the nerve that supplies blood to that muscle and benzodiazepine withdrawal. Some medicines like antihistamines can also cause this twitching. Excessive drinking of caffeine based drinks can also have the same effect.